Walkthrough & Tips to Defeating Sans in Undertale

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Survive and beat the most extreme Undertale boss fight
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Are you stuck trying to beat Sans in Undertale? While Sans has only 1 health, 1 defense, and 1 attack strength, he is still one of the most difficult bosses in gaming. Read on to find out how to beat Sans in Undertale.

How to Sucessfully Beat Sans in Undertale

  1. Equip yourself with Cloudy Glasses and Torn Notebook to reduce damage taken.
  2. Memorize his moves so you can avoid damage and deal your own.
  3. Heal yourself after a dodge and before the next attack comes.
  4. Don't spare Sans; he will try to convince you he's good.
  5. If at first you don't succeed, try again; you may have to battle Sans multiple times.
Section 1 of 2:

Fighting Sans

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  1. There are a few rules you need to know before fighting Sans. They are as follows:
    • The only way to progress in his fight is to attack. If you use an item, spare or "Check" Sans, the battle will not progress and he will attack you with his next attack. This will make phase 1 longer.
    • Whenever you get hit by one of Sans' attacks, you take Karmic damage. It's like poison damage. It will continue to drain your HP and can be very devastating after just getting hit once.
    • Have you ever noticed Sans swaying from side to side during the fight and remembering that skeletons can't breathe? It is possible Sans is vibing to his own song.
    • There are no invincibility frames during the Sans fight. You will continue to take damage from an attack until you move to safety. [1]
    • Sans puts your SOUL (or your heart) into two different modes: Blue SOUL mode and Red SOUL mode:
      • The Blue SOUL mode puts gravity onto your heart. Whenever you jump, you're going to come back down. The protagonist must jump and use platforming techniques to avoid attacks. The height of your jump is dependent on how long you press the up arrow key (or whichever direction you need to jump).
      • The Red SOUL mode is the opposite of Blue SOUL mode. Gravity is not applied when your heart is red. You will float during the Red SOUL mode and can move in any direction. Red SOUL mode isn't used as often as Blue SOUL mode.
  2. You should use the Cloudy Glasses you can buy from Gerson as armor here, because they make Karmic damage drain slower and every few frames, damage is ignored. The Burnt Pan you found in Hotlands gives you 4 extra HP when you use a healing item, but the Torn Notebook causes Karmic damage to drain slower and ignores damage as well. This effect stacks with the Notebook and Glasses. The Empty Gun and Ballet Shoes have a fast attack animation and aiming minigame, allowing you to save a few frames where Karma would otherwise drain HP. The Toy Knife also has a fast-aiming minigame, but a normal animation. The Stick and Real Knife have no effect. The best weapon here is the notebook because it reduces quite a bit of damage that adds up, but you could probably beat him with the other weapons if you wanted to. You'll also need some powerful healing items. The optimal set-up is as follows:
    • ARMOR: Cloudy Glasses
    • WEAPON: From best to worst: Torn Notebook, Burnt Pan, Ballet Shoes, Empty Gun, Toy Knife, Real Knife/Stick
    • Butterscotch Pie (92 HP)
    • Instant Noodles (90 HP)
    • A Face Steak (60 HP)
    • 3 Snowman Pieces (45 HP each)
    • 2 Legendary Heroes (40 HP each), but you can swap 1 for a Sea Tea that boosts your speed (10 HP). You heal less HP this way, but this is helpful for the blaster attacks.
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  3. Sans has 24 attacks that come in two parts. Part 1 is the easier part. Once you learn the patterns, you may be able to get through Part 1 without taking damage The Part 1 attacks are as follows: [2]
    • Strongest Move: This can catch many new players off guard because Sans is the only enemy in the game that gets the first attack. Be ready! It will always be the same move. Once you learn how to dodge it, you can dodge it as many times as you need.
    • Bone Slam: This is part of the Strongest Move attack. Sans starts by putting you in Blue SOUL mode and doing a "bone slam." He will throw you into the bottom of the screen. Then a red box will appear with a sound. This indicates you need to jump. Jump to avoid the bones at the bottom of the screen.
    • Bone Wall: This is the second stage of the Strongest Move. Sans will put you in Red SOUL mode and a series of bones will come at you from either side. You will need to press up and down to navigate through the passageway in the bones the bone wall.
    • Gaster Blasters: This is the third stage of the Strongest Move attack. Gaster faces will appear at the sides and corners of the box and blast you with lasers. You will be in Red SOUL mode. You will need to move to the center or sides to avoid the attack. You'll want to move a little before the attack starts to avoid the attack. Move using the following pattern to avoid the attacks: Center, any side, center again, then top or bottom.
    • Jump rope: During this attack, you will be put in Blue SOUL mode. A series of bones will come at you from one side or the other with a small gap in between each bone. Use small, controlled jumps to jump through the gaps. This attack comes in a variety of iterations with the gaps at different heights throughout the fight.
    • Blue Bones: During this attack, you will see a large blue bone followed by a small white bone come at you. You need to wait until the blue bone passes you and then jump over the short white bone while moving in the same direction as the white bone. If you move while the blue bones pass you, you will take damage.
    • Right and Left Solo: During this attack, you will be put in Blue SOUL mode. A series of short bones will come at you. You will need to jump onto a series of platforms on top of the short bones to safely move across them. Avoid the large bones that come from the top. Another large bone will come at you from the top at the end. The bones and platforms may come at you from the right or the left.
    • Bone Platforms: During this attack, there are a series of platforms with bones in front and behind that move across the screen. The one at the bottom moves left, and the one in the middle moves right. You will be in Blue SOUL mode. You will need to jump and move in a circular motion to avoid the bones. Jump on the bottom platforms going right, then jump down and move left to stay in front of the bones. Jump on top of another platform and move right, when safe to do so.
    • Monorail: This is similar to Bone Platform, except the platform moves right and then left. There are also a series of bones with gaps moving up and down in the middle. You will be in Blue SOUL mode. The easiest way to get through this section is to stay on the platform and just move left or right to avoid the rising and falling bones.
    • Gaster Platforms: During this attack, there are three sets of platforms moving left at the bottom and top, and moving right in the middle. The Gaster faces will appear at different levels and blast lasers. You will be in Blue SOUL mode. Jump on the platforms and avoid the Gaster blasts.
    • Bone Pockets: During this attack, large bones come at you from the left while small bones come at you from the right. You will be in Blue SOUL mode. The best way to get through this attack is to make large controlled jumps over multiple small bones while staying in the space between the large bones.
  4. You're not going to always dodge Sans' attacks even after all that practice. At this point in the game, you should have a couple of healing items. If you're going to heal during the first wave of attacks, be sure to do if after a move that you know you can always dodge. Healing doesn't progress the fight. It just repeats the last attack.
    • Another good place to heal is during the Sans spare in between Part 1 and Part 2. During this spare time, you can heal all you want and nothing will progress until you hit Attack. The last place you should heal is right before his final attack, which is right after his Slam attack.
  5. During the break between Part 1 and Part 2, Sans talks to you and says you can still be a good person, and he gives you the option to spare him. Don't go for it! He will instantly kill you once you do.
  6. After the Sans spare break, the attacks start to get a little more difficult. Get ready for the next set of attacks. They are as follows:
    • Clips: The first attack in Part 2 is a series of 4 quick attacks that require you to react quickly. You may need to jump to avoid bones, move to the side to avoid Gaster blasters, traverse the gap in the bone wall. It's similar to Part 1 attacks, except this time you have to move quickly.
    • Menu Attack: When you see the menu after the first attack, notice there is a bone that moves in and out of the menu. Time your button presses to avoid the bone because it will damage you. It can't kill you, but it can bring your HP down to 1. Later on in the fight, you will see bones in the menu options at the bottom of the screen. Highlight different menu options to avoid the bones.
    • Guided Gaster Blasters: For this attack, Gasters will appear on the screen and aim a laser directly at you. They only fire one at a time, so watch the order they appear on the screen. You will be in Red SOUL. Move in a large oval or circle while avoiding the laser blasts. Later on in the fight, the lasers will get larger.
    • Up and Down Bones: During this attack, one set of bones moves down on the left and another set of bones moves up on the right. You will be in Red SOUL mode. To avoid them, you will need to move in a circular motion in the center to avoid the bones.
    • Slam: This is similar to the bone slam at the beginning of the game. The difference is this time you will be slammed multiple times against multiple sides of the box. You will be in Blue SOUL mode, but gravity will pull you in different directions. Immediately jump from the wall you just slammed against to avoid the bone wall.
    • Special Attack: This is Sans' last major attack. It consists of 4 quick Bone Slams in Blue SOUL mode, immediately followed by Up and Down Bones in Red SOUL mode. Then you will have to navigate through a large wall of bones in Red SOUL mode that moves very quickly. Move up and down to avoid the bones. Eventually, you slam into the wall on the right in Blue SOUL mode. Immediately jump left to avoid the bone wall. Then there are four more wall slams in Blue SOUL mode. To avoid them, jump up, then diagonal down-right, diagonal up-left, then right. Then Gaster blasters form a circle and blast you from all angles. You will be in Red SOUL mode. To avoid them, move in a quick circle in the middle. It's easy to get outpaced, but after this attack, the fight is basically over.
  7. If you have survived the special attack, you are past the point of the fight where you can die. You can relax now. Sans's final attack is to talk you to death. There is going to be a lot of dialogue. Take the time to read it. Do not move. If you move, he will blink and you will be moved back to the center. Just let him talk. Eventually, he will fall asleep.
  8. Once Sans falls asleep, you can push the box to the left by moving left. Then push the box down to the "Fight" option in the menu and select it to deal a final blow to Sans. Congratulations! You win!
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Section 2 of 2:

Tips & Tricks

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  1. You might not beat Sans the first few tries, but keep trying.
  2. Most moves from the boss happen in a specific order, but there is some RNG involved.
    • Figure out patterns so you can reduce how much damage you take.
  3. Many of these sites come and go, but https://badtimesimulator.io/ is a reliable practice site that you can use outside of the game to work on your technique.
  4. They are one of the best sets you can get since they offer you invulnerability. [3]
  5. It's his last attack, and holding the right arrow key will let you glide through a lot of bones.
  6. By doing this, you will automatically move away from the bone walls.
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Community Q&A

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  • Question
    I was fighting Sans and nearly beat him, but then he lasered me a million times and all my health dropped to 0. How do I not get lasered by Sans? He's too hard, even with cheats.
    Community Answer
    Try using an computer emulation of the Sans fight so that you can practice before the real thing. Try to memorize his battle pattern and always pick attack so that it progresses.
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  • Question
    Is it a good idea to fight Sans or do the genocide route in general?
    MATTHEW SCHIFSKY
    Community Answer
    If you want to see the alternate timeline, want to beat all routes 100%, or beat Sans for the thrill of it, go ahead. If you're into the game for just the community and would just rather watch a YouTuber or someone else do it, do that instead.
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  • Question
    I did the scratch Sans fight and I did not have any items. How do I beat him, especially at the beginning with the bone smash?
    Community Answer
    Try to dodge his attacks. If it fails, try to move away from the bones and blasters.
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      Tips

      • In the platform attacks, consider the jump height. Before using items, make sure you have no Karma damage left or after you eat Karma will still lower your health.
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      Tips from our Readers

      The advice in this section is based on the lived experiences of wikiHow readers like you. If you have a helpful tip you’d like to share on wikiHow, please submit it in the field below.
      • There's actually better weapon/armor you can use in this fight than the Burnt Pan; there's a hidden mechanic only in Sans' fight where equipping the Torn Notebook and Cloudy Glasses. Equipping the Torn Notebook gives you invincibility every 4 frames and gives you a 50% chance that KARMA drain is slowed down, and the Cloudy Glasses gives invincibility every 5 frames and has a 2/3 chance to make KARMA tick slower; equipping both gives you invisibility every 3 frames and KARMA always drains slower.
      • INV doesn't matter in this battle. However, Karma works in a different way than you think. First of all, people say using the cloudy glasses doesn't help, but that's not true. They give 1/30 (a Thirtieth of a second) more INV than normal. However that is not quite useful, since INV is very useless in the battle. But if you feel like the battle is gonna be easier with it, then go ahead. You will almost not see a difference in the battle with the glasses, but they will be barely helpful.
      • Here's how you dodge his final attack: press Up > Down > Left > Up. Move in a circle to dodge this attack. Dodge the next bones by going up and down. Next, go to the very middle and press Left > Up > Up > Right > Left > Up > Right. Next, move in a circle to dodge the Gaster Blasters. Try to stay as centered as possible.
      • When the "battle box" extends to the right in part 2, he'll start with a bone attack which you need to go in a wavy pattern to avoid damage. The nanosecond the attack pops up, press and hold "up" to skip the entire "wavy" motion attack.
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      References

      1. ↑ https://www.youtube.com/watch?v=JXb1_7LzGzs
      2. ↑ https://aminoapps.com/c/undertale/page/blog/how-to-defeat-sans-guide/4d2S_YuNQq7BovM6lG8aLeDx3RRgK6V
      3. ↑ https://undertale.fandom.com/wiki/Cloudy_Glasses

      About This Article

      Article Summary X

      1. Stock up on healing items and equip the Burnt Pan.
      2. Jump to avoid the bones during the slam attack.
      3. Navigate through the space in the Wall attack.
      4. Use short controlled jumps to jump through the gaps in the bones.
      5. Do not move when the blue bones pass.
      6. Jump on the green platforms to avoid the bones.
      7. Move out of the way of the gaster blaster lasers when they appear in or around the box.
      8. Attack Sans to progress the battle.
      9. Only heal during the halfway break or after an attack you can dodge.
      10. Wait for Sans to finish talking at the end.
      11. Select Fight after Sans falls asleep.

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