beginning music from undertale battle music of phase 2 is slowed down SuperSonic from Geometry Dash WARNING! Do not heal on the last turn, or else he will get ticked off and attack again, even though he's tired!
When he gets tired, if it starts getting buggy, mercy should work. I don't know how that works, but it just somehow makes the fight script work without soft-locking. Update: It's completely unpredictable. Sometimes it works, sometimes it doesn't. You just have to be lucky, I guess. Update: Hopefully fixed it by shortening the text. If it is still glitchy then IDK.