Who would win in a fight: Sans, or Undyne the Undying?
I think most people would probably say Sans, since he’s the final boss of the G. run, and most people would probably say his fight is harder. Sans’ attacks are big and intense and his powers even extend to the point of being meta. And because he’s more popular. (That’s a factor too in how people think about stuff like this, and I’m no exception.)
However, given the differences between Undyne and the player, I think Sans would have a much harder time fighting Undyne than he did Frisk/Chara. The two big differences are that Undyne has a lot more HP than the player, and her attacks are a lot harder to dodge. I added it up and Undyne the Undying has over 20,000 HP. That is a LOT of HP. Frisk/Chara, when fighting Sans, only has 99 HP. Undyne has over 200 times more, and her defense is nearly as high (90, if I remember correctly). Even if Undyne fought in her usual way and didn’t dodge any attacks, it would take a loooooong time for Sans to actually bring her down to 0 HP. And that whole while, Sans would die if he were hit with even a single attack.
So basically, what this fight depends on more than anything else, is if Sans, who does get exhausted fairly easily and only has 1 HP, would be able to hold out and continue to block/dodge every single one of Undyne’s attacks during the long time it would take to bring her down to 0 HP, without ever even getting hit once.
Personally, I don’t think he could do it. Sans’ powers/attacks are a lot more versatile, intense, and hax than Undyne’s, but Undyne the Undying is too much of a tank. I think he would run out of energy to block/dodge her attacks well before he could take her down. That’s my 2 cents about this imaginary match-up at least!