Board Thread:General Discussion/@comment-37750220-20181214063221/@comment-40273301-20190729050933

Oofoofoofoofoofoofoofoof wrote: But then, given how much damage you deal (enough to kill Undyne several thousand times over), isn't a "But it refused" likely?

Undyne can’t “Refuse” death again after the Undying form. She tried when the player killed her in waterfall, but she just turned to dust.

Actually, she might not even go to her undying form at all if she was in sans place, since there’s noone left to protect after her but Asgore. That’s literally like, 1 person. And since she doesn’t know about the reset mechanic, she’ll think her people are lost forever, and lose her determination, allowing you to kill her with one blow, like mettaton NEO.