I will point out, Sans is only strong because he uses your Karma against you, and takes away your invincibility frames. If fighting someone with no negative Karma, he can only deal one damage per bullet (bone.) Sans is also only able to hold his own against you because he can dodge. Undyne, with her ability to turn a SOUL green (which renders the opponent motionless,) can stop him from dodging, thereby allowing her to win in one hit.