As much as I don't normally like going off of raw game statistics, Sans' damage on KR is directly based on the pain and suffering caused by the target (in game measured by kill count). Outside of this Sans' attack, while ignoring invulnerability frames, deal one damage.
Undyne the Undying has around 24,000 HP, which is almost 2 1/2 times the amount of health that Omega Flowey has.
Undyne may give people a hard time, but she is overall benevolent (towards monsters). There isn't much room for her to -cause- pain and suffering while trapped in the underground.
That said, it is unclear how long she can maintain that form. High levels of determination causes monsters to melt away and deteriorate, as seen in the true lab with the amalgamates and in her death speech. It would come down to whether she could tire Sans out before she disintegrated.
I actually think that regular non-genocide route Undyne might actually have an easier time at this. She 'only' has roughly 1,500 hitpoints, but she doens't have to worry about melting due to determination before she could even land a blow on Sans. As an added benefit, it is shown during a neutral route kill that she is still able to persist beyond death for a short time due to determination (in game 5 rounds, it doesn't matter how much damage the player's attacks deal).
https://youtu.be/tpR_mvYUI8U?t=5m38s
I would have to give this one to Undyne due to power incompatibility with Karmic Retribution. Sans would put up a good fight, but he wouldn't have the damage in this match-up.