I'm trying to beat Sans on Genocide. Needless to say, he's dunked on me for the 89th time at the time of writing this. I have the Burnt Pan and an inventory full of Legendary Heroes and one Steak. And sometimes, when he pulls you down and spikes his bones up, I still get skewered. Any advice?
Here's the ideal item list you should have:
- Butterscotch Pie
- Mettaton Steak
- Instant Noodles (You can only take one IIRC)
- Three Snowman Pieces
- Two Legendary Heroes
The ATK of your weapon doesn't matter at all.
When Sans slams you down into the bones, hold the direction opposite to the one he slammed you. You'll jump out of the way. It's sort of like him flipping the gravity of your SOUL. Note that you'll need quick reactions to actually dodge this as the attack gets faster later on.
Learn to be quick in menus. If you've gotten far enough into the fight, he'll start attacking your SOUL cursor in the menus!
Using an item will not advance his dialogue, and thus, won't advance the fight. Attacking him is what triggers the next line, so use as little items as you need to. This is why the Butterscotch Pie is crucial - it maxes out your health in a single turn.
You're on the right track for having the Burnt Pan. If you have any of the Snowman pieces, Equip them. Equip the items mentioned in the other answers "Butterscotch Pie, Face Steak, Snowman Pieces, Instant Noodles, etc.".
A personal recommendation is to have a Sea Tea on hand. Use it in the second act of the fight, when Sans spares you and gives you time to use items without consequence. It helps you avoid some of the faster attacks, especially Sans' Final Attack.
Also, another tip is to wager how long you can go without using any items. Try to extend your itemless periods as much as possible.
Lastly, Practice, practice, practice. It's the only way. Hope this helps.