Creating a mechanic based on plot armor

I'm writing a dumb little mini campaign where high school kids get essentially isekai-ed into a TV world controlled by a wizard/sorcerer/magic dude who learned about Earth television and became obsessed with it enough to create it himself. The campaign is filled with every TV trope I can think of, and I want the final fight to basically be "invincible final boss until some plot armor bs happens". Any ideas on how I can make plot armor a mechanic in a way that isn't just doing more damage?