((UNDERTALE SPOILERS))

It’s a battle that if you want to mercy her, you just have to survive long enough and the battle will end on its own.

HOWEVER: You can end the battle on the first turn if you saved a Spider Donut or Spider Cider from the ruins- eat it on your first turn and she’ll realize you don’t hate spiders and you can skip the battle. Otherwise, you just have to survive until a spider from the ruins walks in and lets her know you’re not a dick. ovo;