Wanted to leave a little comment, Monster Physiology page explicitly says attacking the soul takes physical durability into account, so an infinite durability gap would mean they take zero damage:
CRT that it links to:
So yeah, the standard durability negation they use wouldn't help them at all in this battle as the gap is uncountably infinite. Sans other dura neg option is via Karma, which can't even kill the player, it just slowly lowers their health overtime (And it even acts as a second health bar at that, as you can't die from Sans UNTIL the purple bar depletes first).
So the standard durability neg via attacking the soul does zero damage, and his other method via Karma can't kill.