Uhh if you have the tough glove and a few punch cards tear up the punch cards for more damage and that’s how you do it well at least that’s how i did it
I restarted my geno run to get Spider Cider to help beat Undyne The Undying. After that i just kept attempting the fight until I won. If the yellow/inverse spears from the green soul sections are giving you trouble there's a guide somewhere online
