Can you spare Undyne the Undying during a genocide run

You can't spare Undyne the Undying, but you can spare monster kid, which ends the genocide run, places you back on the path to a neutral ending, and gives you the normal Undyne fight in 2 rooms.

Short answer: No. Long Answer: No, it is not possible to spare a genocide exclusive boss. Pressing [[X MERCY]] and SPARE multiple times won't progress Undyne the Undying's fight ANY further, due to the fact that she's a genocide exclusive boss. She's protecting all of the underground never giving mercy. You cannot FLEE, nor [[X MERCY]]