After Sans Fleed AGAIN from you, he entered waterfall and doing his third killing spree, he's about to kill undnye, suddenly you killed undnye out of nowhere, encountered him again, and now he's more powerful

Sans: oh. You killed the fish before me, didn't you??

Sans; summons a spear how about try I kill you, AGAIN

Sans Prepares his spear, wants a second rematch

What do you do???