Boss Undertale: The Complete Guide 2026

Undertale has a a dozen or so of the main boss units, and about fifteen, counting the mini-bosss, spread across 7 zones.

In this article, I'll introduce you to the full list and explains how to beat them.

This video game is in English by default, but you can put Undertale in French.

Attention : Major spoiler risk if you haven't played it yet.

Complete list of boss on Undertale

In Undertale, each confrontation can be resolved in two ways: you can go to the monster or choose to’save. This choice determines the road that you follow.

• The Neutral Route It combines fighting and dodging without really favoring one over the other.
• The Pacifist Road forces you to spare everyone, from Toriel at Asriel.
• The Genocide Road turns easy battles into brutal trials, with Sans as the final wall.

This is the Complete list of boss, sorted by region. Click on a name to go directly to its strategy.

Ruins (Ruins)

The Ruins are home to a mini-boss and one main boss. First, you come across Napstablook, then Toriel before leaving the area.

Napstablook

Napstablook is the first mini-boss Undertale. It’s a sleeping ghost you encounter deep within the Ruins.

Talk to the ghost, then chose to push her away forcefully to start the battle. This battle is inevitable, but not difficult.

At first, his tears fall straight down. You have no trouble dodging them by staying in the lower part from the box. When they become rounder, stay close to the center.

Choose to’Cheer for Napstablook 3 times in a row to end the fight without violence. Once the fight is over, Napstablook steps out of your way and you can continue your journey through the Ruins.

Toriel

Kill Toriel

If you decide to kill her, One shot is all it takes. Her final line depends on how you handled her.

For example, if you kill her after sparing her, she'll tell you that you're just as cruel as the rest of the monsters, then dies laughing.

Saving Toriel

If you choose to spare it, the battle takes a while. Toriel has four different attacks :

1. Fire falls from the top of the box: Stay in the middle to dodge.
2. A propeller of fire spins downward: is still in the center.
3. One-handed swipe: Position yourself in the lower center, then move along the edges as the flames follow you.
4. Two-handed sweep: Start in the center, then move to the sides.

To note : Toriel doesn't really want to hurt you. If your health is low, her attacks become less precise on purpose.

By constantly sparing her rather than attacking her, she eventually starts talking to you. Keep being nice to her, and she'll let you through.

Go through the doors, walk down the long hallway, and you'll find Flowey, which tells you whether or not you've killed someone before taunting you.

Go out through the large doors, and you'll enter the next area: Couveneige.

Couveneige (Snowdin)

Couveneige (Snowdin) is the second zone from Undertale. There you run into several mini-boss, all members of the royal guards, before facing the region's top-ranked boss.

Doggo

Not far from the entrance, you spot a large box sitting in the snow. As you get closer, a dog comes out : It is Doggo.

He tells you that he can only see things that move, which immediately triggers the fight.

The trick with Doggo : to remain completely still. His blue spears won't hit you as long as you don't move.

Saving Dogo

1. Select «Pet» (Pet) : his name is listed under yellow and then you can spare him.
2. During the attack on Doggo, stay still so as not to be hit.

Kill Doggo

1. Attack Doggo and stands still in the face of his counterattacks.
2. It is generally necessary to two or more attacks to finish it off.

Remark : You can still pet him to unlock a special dialogue before killing him.

After this altercation, continue your journey eastward.

Dogamy & Dogaressa

A little further on, two dogs holding axes are blocking your way.

These are Canisse and Canibelle (Dogamy & Dogaressa), another pair of royal guards. The fight breaks out because of your smell.

Their attacks take the form of spinning blue and white hearts. Dodge them, or position yourself where the blue hearts are about to land and remain motionless.

Kill Dogamy & Dogaressa

It is generally necessary to at least two shots at each dog. Once one of the two is defeated, the other becomes much more aggressive.

Saving Dogamy & Dogaressa

To save Canisse and Canibelle, you must do one good deed after another in the right order:

1. Start by «Roll» to change your scent.
2. Then choose «Sniff» so the dogs can get used to your new scent.
3. Finally, pet both dogs so that I can spare them.

Attention : Between caresses, they launch a major axe attack harder to dodge. Stay focused.

Surprised that a dog can pet other dogs, Canisse and Canibelle leave you continue south.

Greater Dog

After passing through an area filled with snowballs, one of them suddenly blocks your path.

As soon as you get close, Maxichian (Greater Dog) appears : He is the last of the Couveneige Royal Guard dogs.

Once you're close enough to that snowball, Maxichian (Greater Dog) will appear.

Maxichien attacks are two different types :

• The first attack is a spear that moves across the screen from white to blue. Try standing still and letting it pass when it's blue..
• The other attacks are «barks» from a dog that's dozing, rStay high and keep moving to dodge them.

Kill Greater Dog

If you decide to kill Greater Dog, you have to attack it two or three times to end the fight.

Saving Greater Dog

On the other hand, to spare him, you must act in the following order :

1. Beckon (waving)
2. Pet (Caresser)
3. Play (Play)
4. Pet (Caresser)
5. Pet (Caresser)

By performing these actions correctly, you'll be able to save Greater Dog.

Once you've defeated Maxichien, you can continue eastwards across a long bridge.

Papyrus

Papyrus is the boss this zone's main, and you'll have to fight him if you want to leave the town of Couveneige (Snowdin).

His first attack will make you blue soulwhich means that it is now affected by gravity and will sink to the ground.

You can jump and move using the arrow keys, so use them to dodge his attacks.

Remark : This battle might be tough, but if you lose, you won't die, and you can come back and try again. If you lose three times, you can skip the battle.

Killing Papyrus

Papyrus has a lot of HP, and you'll get time to kill it or spare it.

If you do that to him lose HP, he'll offer to spare you instead. It's up to you to decide whether you want to spare him or kill him.

Save Papyrus

Flirt with him and keep going easy on him until all his attacks are complete.

Once the battle is over, Papyrus will ask you a question before letting you through.

The Falls (Waterfall)

In Undertale, the Chutes (Waterfall) have only two boss, but the battle against Undyne will come up several times as you play through the game.

Undyne phase 1

As you make your way through the area, You'll come to a pier. You'll suddenly be attacked by a spear, and Undyne will appear.

You'll then engage in a battle with her, during which you'll have to dodging spears in the combat phase and along the pier.

Keep heading east. If the spears hit you again, you'll return to combat mode for one of its attacks.

The best way to dodge the spears is to keep heading east. Go to Be careful when moving up or down, because you can easily end up in the path of the spears.

You'll find reeds again in which you'll be able to hide and you won't be able to move during a short scene.

Attention : The following area is a save point, so don't forget to save before continuing.

Undyne phase 2

Further on, you'll find yourself on a pier again. This time, the ground will light up and spears will rise up from these points.

Keep moving in a general southeasterly direction, but your path will be somewhat determined by Undyne's attacks. You'll eventually reach the end of the pier, and the jets will stop.

Turn around and retrace your steps so you'll face Undyne before falling into the depths.

Mad Dummy

As you try to get out of the sewers where you fell, you'll be stopped by a training dummy.

Remark : No matter what outcome you're aiming for, the method is the same.

Mad Dummy will attack you through the attacks of her model friends. The projectiles will follow you, which means you'll be able to direct them as much as possible toward him in order to deal damage.

Keep in mind that rocket-type projectiles must spin once before they can fly off to strike their target Dummy, SO guide them accordingly.

Once he has taken enough damage, your old companion Napstablook will appear and put an end to the battle.

Follow Napstablook north to have another chance to save before continuing.

Undyne

boss's battle with Undyne takes place at the end of Chutes level, when you get to the mountain.

After the cutscene, you'll be able to save your progress before fighting Undyne.

Undyne will make your green heart, which means it can't move. Then it will shoot arrows at you.

You have a blocking capacity in the form of a blue line. This blue line must be aligned with the direction of the arrows so that you don't take any damage.

Sometimes arrows are yellowwhich means they come from the opposite direction.

When your heart is red, you must act as usual and dodge the spears, just like during your previous encounters with her.

Killing Undyne

Constantly attack Undyne and don't choose to flee. Once her health drops to zero, she'll be determined to keep attacking and try to survive.

His attacks will become slow and easy to avoid, and you'll be able to finish him off.

Save Undyne

Once your heart is red, you have the possibility of escape and it will chase you, so hurry back into the tunnel.

Undyne will catch up to you, and you'll have to survive his attack so you can run away again and keep running.

Papyrus will call you Meanwhile, don't worry—Undyne won't come near you while you're on the phone.

After a long horizontal corridor, you'll come to a desert area.

If you want to focus on the Peaceful Route or befriend her or Papyrus, you must bring him a cup of water from the cooler.

Undyne will get up and leave immediately after you've done it.

Remark : Whether you kill her or spare her, you'll go through a similar number of rounds.

Calciterre (Hotlands)

In Calciterre (Hotlands), you'll meet some boss, the most notable of which is Mettaton. You'll have to face him several times in different ways.

Mettaton test 1

Your first encounter with Mettaton will take place in the laboratory, while you're talking to Dr. Alphys.

Just as she's explaining to you that her robot might cause problems, it appears in the room and starts some sort of quiz battle.

the Dr. Alphys will show you the answers with her hands, so keep an eye on her.

The answers are: D, C, B, D, A, D, any and C.

For the last question, the Mettaton's speech and Dr Alphys' reaction will change depending on the answer you choose.

Remark : During your turn, your actions will have no effect on Mettaton.

Once the battle is over, Mettaton will leave and the Dr. Alphys will update your phone and will sign you up for a social network, whether you want to or not.

Continue eastward and leave the lab.

Mettaton test 2

After a series of puzzles and updates from Dr. Alphys, you'll eventually arrive at a dark area.

Dr. Alphys will hack the lights to turn them on, and you'll discover that you're in a cooking show with Mettaton.

First of all, you need to rGather the ingredients behind you.

Tip: You can carry them all at once, so take them all and bring them back.

Let the scene play out for a moment, and you'll end up with a jetpack to climbing an incredibly high piece of furniture. Your goal is to obtain a substitute human soul extract.

A timer starts, and you have to dodging ingredients that Mettaton drops above you. Don't panic—you have plenty of time to reach the summit.

Mettaton will leave at that point, and you'll be able to continue east to a new save point.

Royal Guards

Before facing Mettaton again, you'll be interrupted by Royal Guards (Royal Guards) that are supposed to protect you from humans.

At least that's what was supposed to happen until they realized you were the Human and started a boss mini-combat.

Although not difficult, this battle with the Royal Guards is quite specific. It will be difficult for you to find the right combination to save them quietly.

Choose to’act on RG 02 first to clean its armor. This will present a long vertical combat zone with a floating piece of green armor.

Remark : If you don't do it often enough, you'll have to clean it again.

After cleaning the armor, you must cChoose RG 01 and select the "Whisper" action..

He'll confess his feelings to R2, and then you'll be able to spare the Royal Guards so they can go for ice cream, which will put an end to the battle.

Feel free to go back to the save point to regain some health, otherwise continues all along the way to another dark room.

Mettaton test 3

After Dr. Alphys turns the light back on, you realize you're in another Mettaton TV show. This time, it seems like you're a reporter on the scene.

Once you're ready to take action, Move to the first object you find. If you go a little to the right, you'll find a basketball.

Remark : Any object will do, because they're actually bombs. A timer starts, but don't worry—it can't count down to zero.

Now you must defuse all bombs with Dr Alphys' application that was installed on your phone.

Interact with an object, then Press the «Defuse the Bomb» button when the object is in the zone and turns green.

A good way to defuse them all at once is to follow this path :

1. Head down and then turn left after the lasers to defuse the bomb.
2. Then retrace your steps and Turn right to get on the treadmill to disarm the script.
3. Keep going straight ahead and jump to defuse the gift.
4. Jump to the left to defuse the basketball.
5. Finally, return to the center to neutralize the glass of water that Dr. Alphys stabilized right there so you could finish the job.

Mettaton will leave once the operation is complete, so you'll be able to keep heading east.

Muffet

After buying some spider pastries, you'll come to a hallway full of spiders where you'll be trapped in a thick web that Muffet has set for you.

This battle can take place from different ways, depending on what you've done so far.

• If you've spent enough money to buy an arachnid treat in Calciterre, you won't even have to fight—you'll simply be set free.
• If you still have your treat from the Ruins, decide to take action and eat it right in front of her. She'll accuse you of theft until she receives a telegram from the Ruins Spiders, which will end the battle.
• If you don't have a treat, your battle against Muffet becomes difficult if you choose the Pacifist Route.

Muffet will make your purple heart, which means you'll only be able to move by holding onto one of the three lines.

Muffet owns three types of attack. She will send :

1. Regarding the spiders along these lines to attack you.
2. Regarding the doughnuts that will bounce around your combat zone.
3. Regarding the croissants that will come back like a boomerang.

Sometimes, his pet appears and keeps pushing you upward, while you still have to avoid the spiders.

Once you've managed to avoid your pet during the third round, Muffet will let you go, and the battle will end. Otherwise, you can keep fighting to defeat her.

Keep going to the east.

Mettaton test 4

You'll come to a sort of castle and Mettaton will appear. He'll start singing a romantic song about how you have to die before you let yourself fall into a hole.

For the next puzzle, Dr. Alphys will be on hand to help you at the last minute, no matter how successful you are, Mettaton will fight you.

Dr. Alphys will call you in the middle of the battle and tell you to’Press the yellow button on your phone.

Select the "Action" option (Act) to do it, then fires several shots at Mettaton and he'll run away.

After another telephone conversation with Dr. Alphys, continues eastward, then northward to reach the MTT hotel.

The Core (The Core)

There is only one single combat of boss who's waiting for you there—and it's none other than Metatton.

Mettaton

Before facing Mettaton, don't forget to save your progress.

Mettaton will reveal something rather strange about Dr. Alphys before train for a fight where she won't be able to help you.

At the start of the battle, you'll have a yellow heart who is shooting, but your bullets won't damage the robot at all. Instead, you can use them to block its attacks from hitting you.

You can only shoot at the squares with a circle to destroy them or on yellow objects to move them.

Dr. Alphys will call you to’inform of the presence of a switch in Mettaton's back.

Select «Action» and then «Turn Around» to convince Mettaton to turn his back on you, then Click the switch.

Mettaton EX

After pressing the switch, Mettaton will become Mettaton EX and a graph will appear on the side, showing the Audimat.

Remark : If you die during your battle against Mettaton EX, you'll return to the moment it transforms.

Your goal is to boost the audience to 12,000, and there are several options for achieving this.

• Show off (Boast) is a good choice if you're confident in your ability to dodge attacks. This action will also allow you to increase your Audience Rating more quickly.
• Put down (Pose) is a good starting point that will help you steadily improve your rating.
• Playing the Bad Guy (Heel Turn) will earn you points every time you take damage.
• Tackle will let you earn about 300 points each turn.

To dodge his leg attacks, you must:

• Pulling on white legs to get them moving.
• Pull on the yellow legs just before making contact with them.

Another attack sends up bombs and mini-Mettatons. Shooting at the bombs will help you eliminate them. The surviving mini-Mettatons will throw hearts at you.

Remark : If you shoot at a bomb, it will explode in the shape of a large cross, so you'll need to move away from it immediately afterward.

His heart attack triggers flashes of lightning. You must dodge them and pull on his heart to end the assault.

Remark : Be sure to stock up on food so you can heal yourself. The more expensive the food, the better. Every time you eat «brand-name» food, the audience appreciates it more based on its price.

More later, knives will also appear on the sides of your box while he was having a heart attack.

Shoot the disco ball and Avoid moving when the lasers turn blue.

Once you reach 12,000 in Audimat, Mettaton EX will start receiving phone calls from fans. He'll realize that he doesn't want to fight anymore, or that his health is exhausted.

The fight ends and Dr. Alphys approaches, and she'll be relieved that he isn't dead, or she'll act as if there's no problem if you killed him.

From there, you can move on to the next zone.

Mettaton Neo

If you follow the Genocide Road, Mettaton will explain that you are a threat to all life, not just monsters. He therefore concludes that you must be eliminated.

Mettaton se will be transformed into a Mettaton Neo to face you.

Remark : The only «Action» you can perform is «Examine,» which will reveal Mettaton Neo's stats.

To beat it, simply touch it once.

Once the battle is over, you can move on to the next zone.

New House (New Home)

Depending on your background, Nouvelle Maison (New Home) could be your last zone on Undertale. Warning, spoilers!

Asgore

On arriving in the throne room, you'll meet Asgore. At first, he'll have his back to you and ask you to wait while he waters the flowers.

Surprised to see you, he'll eventually smile, and then will ask you to follow him when you're ready.

Remark : You'll have the chance to save your progress several times before facing Asgore.

The first thing Asgore does is to delete your «Clémence» button (Mercy). You have to fight him, whether you want to or not.

You can talk to Asgore three times to reduce its attack and defense.

You can also lower its attack and defense if you still have the Toriel pie with you. Just eat it in front of him.

Remark : Keep in mind that Asgore will never reduce your HP to 1 in a single turn. He'll give you a chance to heal yourself. If you don't heal yourself, he can kill you with his next attack.

Asgore's attacks are very similar to Toriel's, but there's a new twist. some differences and new movements :

• First attack: Flames shoot out of two hands. Dodge them by redirecting the flames and always making sure you have an escape route.
• Second attack:  Flames are swirling downward. Try to stay near the center to dodge them.
• Third attack: A small number of flames will fall, and a warning will appear on each side in the form of a red exclamation point. Make sure to move to the other side before it fills with flames.
• Fourth attack: His eyes will flash according to the color of the scythe before he attacks. This attack can be quite rapid:
  • Blue : Don't move.
  • Red : Move immediately after your previous move to avoid attacks.
• Fifth attack: Another of his attacks takes the form of a ring of fire. It closes in around you, leaving a small opening. Don't stray too far from the center when you escape.

If you decide to’attack Asgore For the first time, you unfortunately have no choice but to touch it.

Remark : A white bar will cross the screen, and the number of bars will depend on the weapon you have equipped.

You have to hit it just as the white bar is in the center to touch it!

Once you've dealt enough damage, Asgore yields. Are you going to spare him or kill him?

Flowey

Everyone's favorite evil flower returns and claims human souls to compete against you in Undertale.

This boss fight is very different from the others. You don't really have any standard actions or a small box to help you move around.

Flowey's attacks fill the screen, and you must Try to dodge them as best you can.

When the "Combat" button (Fight) appears, you must press it. His health bar will appear and he will briefly take damage.

Remark : Many of its attacks are guided by lasers to help you dodge them. But most of the time, you just need to keep moving as much as possible.

When Flowey's screen flashes with a warning and a heart, it means the soul is about to take control. The entire screen then becomes a combat zone.

As usual, dodge the attacks. But stay alert, because a Action" button (Act) will appear and you definitely don't want to miss it!

Tap here for help and you'll receive items that will be very useful during your battle against Flowey.

Remark : If you die, Flowey will bring you back into the game to kill you over and over again. In a way, it's a blessing, because you get another chance to take him on!

After having confronted all human souls, from the care items start to appear during normal fight scenes.

Once you've depleted his health bar, Flowey load an old backup where he's in perfect health and will shoot at you with a super laser, over and over again.

After killing you and mocking you repeatedly, the souls will step in to put an end to him.

You now have the the option of killing or sparing Flowey.

Laboratoire Originel (True Lab)

This zone is available only on a Pacifist Route and contains the real final boss over Undertale.

Asriel phase 1

Flowey turns out to be Asriel, and the battle against the final boss will begin. This battle isn't too difficult, because your actions make it much easier.

Alternate between these actions and use the items as needed to survive:

1. Select «Hope» (Hope) to reduce damage next turn.
2. Select «Dream» (Dream) to stock up your inventory with healing items.

Tip: Heal yourself whenever your HP drops below 10, because it can easily take that much away from you.

Asriel owns several attacks :

1. Starfire (Star Blazing) : A shower of stars will rain down on the battle zone. Stay in the lower corner. The last big star will explode and send out smaller stars, so be careful.
2. Shock Breaker (Shocker Breaker) : Exclamation points will appear right before the lightning bolt—try to keep moving to avoid them.
3. Saber of Chaos (Chaos Saber) : Depending on Asriel's movements, step to the side to avoid his attacks. Then, for the final move, stay in the middle.
4. Chaos Breaker (Chaos Buster) : Stand where the lasers aren't to avoid getting hit. The final move will fire a large beam, so move away from where it's aimed to avoid it.
5. Black hole (Black Hole) : Asriel will fill the entire screen and start sucking up the diamonds. Stay as far away from the center as possible so you don't get sucked in.

Asriel phase 2

Asriel will then say that he only used a small part of its power and transforms into a winged beast.

All you can do for a few turns is "Struggling" and dodging his attacks.

Remark : If you die, your heart will refuse to give up and will regenerate itself.

As the battle progresses, your Action" button changes to "Save" button and you won't be able to choose to save each of your friends.

Each of them will attack you in the same way they did during their respective boss battles, and you’ll have to choosing the right actions to remind them of your friendship:

• Undyne: Small Tape, Recipe and Face.
• Alphys : Encourage, Quiz and Call.
• Papyrus : Casse-crâne and Recipe.
• Sans : Glandouiller.
• Toriel: Câliner and Clémence.
• Asgore: Câliner and Clémence.

Select «Save» one more time once all your friends have been saved, and then you'll be able to save Asriel himself.

Keep «saving» Asriel and dodging his attacks which become less and less intense.

The battle will end and you'll talk to Asriel.

Sans

If you follow the Genocide Road, you won't face either Asgore or Flowey, but Sans.

Remark : You're in for a tough battle, and you'll almost certainly have to try several times before you can beat it.

Sans will mock your many deaths. His attacks are fast and merciless, and he dodges all of yours.

Just like in the battle against Papyrus, your heart turns blue and you'll have to jump over bones, despite the problem with gravity.

One of its attacks is to make skulls appear on either side. Use the platforms to dodge the beams they shoot.

WARNING : Whether Sans If he offers to spare you, make sure to attack him. He'll kill you if you spare him.

Next, a series of attacks awaits you when Sans will threaten you with his special attack :

• A long attack will begin, during which your combat zone will expand to the side, and you'll have to dodging bones.
• Other skulls will appear and shoot aggressively, Move in a circular motion to dodge them.

Next, Sans will launch its special attack, which consists of do nothing.

To overcome Sanssimplywait for him to fall asleep. It will take some time, but eventually it will happen.

Move your combat zone over the «Combat» button, then Click on it twice to kill it.