Thats a tough one.
Considering you got a small skeleton with gravity powers with high ass durablilty vs a guy who was once human and became sort of a demi god.
Well sans doesn’t attack physically, his attacks directly hit the souls, which in sephiroths case, would be the life stream.
But who’s more determined?
Well, Sans is immune to Supernova and Heartless Angel, as one divides the target’s current health by 16, and the other turns the target’s health to 1. Sans, having 1 HP, won’t be affected by either. I don’t know if he could avoid the other attacks.
Well, While sans has only 1 HP cap, he also sleeps a shit ton and raises his health above the max damage of 9999999 and survives the hit. His HP would still technically have only 1 HP though so the fractional attacks wouldn’t effect him. Sephiroth would need to use a good meelee attack