How to beat Muffet pacifist
1: Armor Do you have the Stained Apron and Burnt Pan? If not, go get them. They are the best attack and defence at this point in the game and boost healing. However, if you get hit a lot in one turn or have healing items that heal 20 HP or more, use the Torn Notebook, since you’re on a Pacifist run and damage doesn’t matter.
2: Attacks. On attacks you have trouble with, pay Muffet. Don’t forget to struggle 3 times first so you get a discount.
3: Last resorts. If you absolutely can’t beat her no matter what and it’s ruining the fun of the game, the last options available that I would not suggest in other conditions are Temmie Armor and hacking. For Temmie Armor, since I’m assuming you’ve died a bunch now, it should have a lowered price, probably enough to farm dog residue or cloudy glasses and buy it. (I’d recommend taking it off after the fight though, it ruins all the fun or challenge of the game.) As for hacking, you can look up how to get into the game files and hack your save. The 13th line of your save file is your first inventory slot, change it to 7. This will give you a spider donut. But then, you’d be a dirty hacker.
Otherwise, the only way to defeat her in pacifist mode is to continue dodging her strikes; she will eventually (shortly after the “dinner” phase with the huge spider) receive a note from the spiders in the ruins and cease attacking you. (When it is your turn, a symbol will indicate the next attack she will do. If it is one you are having trouble with, go to the “Act” option and pay money (the amount will increase with each attempt, but it will slow down the next attack.)