Top 10 Hardest Undertale Bosses

Undertale may look simple on the surface, but anyone who's faced its toughest bosses knows that looks can be deceiving. The game is packed with creative battles, unique mechanics, and surprises that can catch even experienced players off guard.

Some bosses are just a fun challenge, while others will have you gripping your keyboard in frustration, desperately dodging a wall of incoming attacks as your health bar shrinks to nothing.

This list is all about the fights that push you to your limits. Some test your reflexes with chaotic bullet hell patterns, while others force you to think fast and adapt to ever-changing mechanics. Some battles are brutal no matter how skilled you are, and others feel nearly impossible without perfect timing and a little bit of luck.

And, of course, there are those fights that get even harder depending on the choices you've made throughout the game.

Now it's your turn to vote for the fights you think deserve to be at the top. Which bosses made you sweat the most? Which ones took you hours (or days) to finally defeat? Vote and see which battles other players agree are the toughest Undertale has to offer.

The Top Ten

Sans

Sans, or Sans the Skeleton, is a character in the 2015 RPG Undertale, created by Toby Fox. He is a lazy, pun-loving skeleton who serves as a supporting protagonist in the "Pacifist" and "Neutral" routes of the game, and a heroic antagonist and final boss in the "Genocide" route. Sans is known for his... read more

I think Sans is easily the hardest for many reasons. His attacks at the start are unexpected and will quickly add a few deaths to your total during the fight. Players will get annoyed eventually, and when people are annoyed, they play worse.
Sans irritates the player by dodging their knife swings. His final attack is very hard to dodge. All of his attacks involve parkour-style dodging and quick movements. Also, during most of the fight, you're stuck using the blue heart, which is harder to control than the red heart.
Those are my reasons why the Sans fight is the hardest. Sorry this comment is very long. I just wanted to point out a lot of reasons. Thank you for reading.
Undyne the Undying

Undyne is a main character in Toby Fox's 2015 RPG Undertale. She is the fish-like, armor-clad captain of the Royal Guard, devoted to protecting the monsters of the Underground. Her name is a pun on the word "Undying" and a reference to the mythical water spirit, "Undine."... read more

I beat every boss in Undertale within 4 or 5 tries. This is all except for Undyne the Undying (who I have been stuck on for weeks) and Sans (who I haven't fought because I'm stuck on Undyne the Undying). Omega Flowey took me about 3 tries and was a really quick boss fight because I'm pretty sure you don't have to repeat phases.
Asriel shouldn't be on this list because you can't die to him. Asgore was the hardest boss in all of the neutral/pacifist route (took me 5 tries). However, he does not even compare to the difficulty of Undyne the Undying. She should be second on this list.

I have been stuck on this fight since the start of December. The time I am posting this is December 22, which is 22 days. The farthest I have ever seen is 75 percent health. One of the first attacks has slow arrows. It seems like it's easy, but it's not because there are so many of them.
Omega Flowey

He almost instakills you with a single hit, manipulates you, and makes it impossible to escape. By sheer luck, they make a mistake and use a super heal on you, which they could easily use on themselves! You also don't even have the option to act! They just have one attack, and they go first instead of you. Plus, they will remember everything!

Although his battle does save, no amount of skill can let you not die on this guy. To be fair, I haven't attempted the genocide route, so I haven't battled Undyne the Undying or Sans, but he is much harder than Asriel, as his battle saves even more.
Asriel Dreemurr

Asriel Dreemurr is a character in the 2015 video game Undertale, created by Toby Fox.
He is the biological child of King Asgore Dreemurr and Queen Toriel, but he was believed to have died before the game's events. In the True Pacifist Route, where the player spares every character, Asriel's backstory... read more

You can't fully die and don't have to restart the entire fight? Nice! So I will dodge attacks and die a couple of times. That's okay and will win, right? Easy boss! Well, that dodging isn't really, you know, and at 20 HP... Anyway, can't 'die' doesn't mean it's easy. Dodging the attacks is a huge challenge.

While you cannot die in this fight, his attacks are immensely hard to dodge and much more difficult than Omega Flowey's. The setback is really annoying as well because the chaos buster almost always decimates your health.
Asgore

Asgore Dreemurr is a boss monster and the king of the monsters in the 2015 RPG Undertale, created by Toby Fox. Asgore is portrayed as the main antagonist, with a plan to collect seven human souls to break the barrier trapping the monsters underground. His character is slowly revealed through hints and... read more

The first time I fought Asgore, I died to him at least 7 times. I just couldn't keep up with the attacks getting faster later on in the fight.

One of the hardest bosses. Worn Dagger is the strongest in this route, but Burnt Pan is the best. It can do 300 or more damage with a perfect hit.

Asgore was pretty hard, to be honest. But his attacks are decently easy to dodge once you learn them, especially the orange/blue axe attack (whatever you'd call it). It took me like 7 tries to beat him. This was pacifist, mind you.
Mettaton

Mettaton is a character in the popular 2015 RPG game, Undertale. He is a robot with a soul built by Dr. Alphys and is the only television star in the Underground. Mettaton is a popular character within the fanbase, known for his flamboyant personality and dramatic flair. His notable features include... read more

Mettaton is the hardest pacifist fight, in my opinion. His attacks are super chaotic and almost impossible to dodge. Some may say Asgore is harder, but once you get his attacks, he's super easy. It took me 8 items to beat Mettaton and only 5 for Asgore. I died 3 times for both but barely beat Mettaton with 2 health, whereas I beat Asgore handily.
The reason why Mettaton is harder than Flowey or Asriel is because you can't lose to them. Anyway, Mettaton is epic.

I think I struggled with this boss fight the most. The bullet shooting is very unique and kinda hard, in my opinion. Probably the hardest boss fight in the game. I know about Sans, but after a bit of experience and learning his attacks, you don't struggle much against him, at least for me. Mettaton EX is hard, if not the hardest.
Muffet

Muffet is a character from the game Undertale. She appears in the Hotland area, where she runs a bake sale to raise funds for the spider clan. Her goal is to help reunite the spiders from the Ruins with those in Hotland, as they cannot cross the Snowdin region.

Muffet got my vote. The way her attacks come at you... Eat a spider donut in front of her to skip the battle, or if you already bought a spider donut in the ruins, in the dinner phase, a spider will bring up a note and the battle will end. Struggle or pay to slow her attacks. Took me many tries.

Muffet is my fave character and super evil and cute. I mean, she can also control your soul like Sans, and every time I see the doughnuts and pastries, I get hungrier, which breaks my concentration.

Muffet is actually pretty easy to beat if you saved the spider donut or spider cider. I had it and ate it in front of her. But if you don't have one, you have to spare her 17 times, I think.
Undyne

Undyne is a main character in Toby Fox's 2015 RPG Undertale. She is the fish-like, armor-clad captain of the Royal Guard, devoted to protecting the monsters of the Underground. Her name is a pun on the word "Undying" and a reference to the mythical water spirit, "Undine."... read more

Waterfall is peaceful without Undyne and Mad Dummy. It should be noted there's a way to skip part of the battle if you get low HP, or even better if you use a skip with a healing item. This skip is not a glitch, by the way.

I had no problems with beating Undyne. She is difficult but fun at the same time, requiring exact timing and being able to move your shield to the right spot.

I struggled when fighting her. It took me about 7 tries. This should be swapped with Mettaton as number 6, with Mettaton at number 7.
Papyrus

Papyrus is a character from the 2015 game Undertale, created by Toby Fox. He is the younger brother of Sans the Skeleton, a Royal Guard in training, and a sentry in the town of Snowdin. Unlike his brother, Papyrus is energetic, loud, and somewhat full of himself, but in an endearing and earnest way.... read more

The only time I had trouble with Papyrus was when I was attempting the pacifist route.

He was only threatening in Disbelief. In regular Undertale, he is a cakewalk.

Slightly harder than Toriel, but still, you can't die, so it's easy.
Toriel

Toriel is a goat-like monster who is a secondary major character in Toby Fox's 2015 RPG, Undertale. She saves the protagonist from Flowey and guides them through the Ruins, of which she is the self-proclaimed caretaker. She is the ex-wife of Asgore, the mother of Asriel Dreemurr, the adoptive mother... read more

Toriel is possibly the easiest fight in the game, whether you're going pacifist, neutral, or genocide. You cannot die in this fight unless you intentionally run into the fire.

Technically, you can't die in this fight, but just try to not get to 2 HP when you are on pacifist. It's freaking impossible!

Strangely, I defeated Photoshop Flowey in three tries, but on my second playthrough, this was hard.

The Newcomers

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Glad Dummy
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Snowdrake's Mother

Snowdrake's Mother is well, Snowdrake's Mother, as well as Snowdrake's Father's wife and an, amalgamate found in the True Lab. It is fused with 16 monsters. It's battle music is a slowed, distorted version of the Snowdin Forest Music. It can morph from a refrigerator to something man-shaped to the image found in the infobox.

The Contenders

Mad Dummy

Waterfall is peaceful without Mad Dummy and Undyne. There's a way to win this in five turns, but this requires certain moves on the enemy turn.

It took me many tries. I just couldn't lead the projectiles to the dummy without getting hit myself.

He took me around 4 or 5 tries to beat, so not really that hard.
Mettaton NEO

Basically Undyne the Undying's fight.
Napstablook

Napstablook is a secondary character from Toby Fox's 2015 RPG Undertale. He is a melancholic ghost and the cousin of Mettaton, who left to pursue fame. Napstablook is characterized by his low self-esteem and depressive personality, often staying at home and enjoying music.

Missed the memo and is the same difficulty as before.

The tears that go around the walls are almost undodgeable unless you spare him.
Mad Mew Mew

This secret boss is actually really tough. The attack patterns are insane, and it makes it worse that you have to use the analog sticks. It's the third hardest fight in the game, in my opinion.
So Sorry

I feel so sorry for So Sorry. He doesn't mean to fight you. He just accidentally ran into you, which started the fight.

This is a secret boss, but it's hard. I'd say it's the sixth hardest boss, right after Asgore.

Harder than Muffet, Undyne, Papyrus, and maybe Asgore, but not as tough as Mettaton.
Greater Dog

This is easy. All the attacks are just barking when you use ACT (in pacifist run). I don't know what happens if you do a genocide run. Maybe you should try it.

Honestly, I was stuck on this for ages during my first run. I was trying pacifist but gave up and went neutral.
Lemon Bread

By far the hardest Amalgamate. I died to Lemon Bread like 13 times the first time I fought them. I beat Sans on my phone easier than this.
Endogeny

Endogeny is a minor character from the indie game Undertale, which was developed by Toby Fox. It is one of the "Amalgamates," a group of monsters that have been fused together as a result of unethical experiments.
This creature appears as a disturbing blend of several dogs and is encountered in... read more
Memoryhead
Reaper Bird
Knight Knight

It was easy for me because I learned Shyren's song by humming five times.
Royal Guards

When you kill one, the other gets stronger. If you don't know how to spare them, it's pretty hard.
Dogamy and Dogaressa

I took so long against these two because of their axe attack. Also, how come there are two axes but only one dog? My brother had to beat them for me.
Lesser Dog

Lesser Dog is a character from the indie video game Undertale. He is a member of the Snowdin Canine Unit and becomes increasingly enthusiastic when the player chooses to pet him. As the encounter continues, his neck stretches longer and longer, creating a humorous and memorable experience for players... read more
Doggo