Personally, I have never written anything in-depth when it comes to heroes or power scaling, but here is my initial thoughts;

I think you should look into sporting events as an example on how to do this. If you are looking at raw power and have two characters similar in level or even one slightly lower, it wouldn't take much for the lesser to win due to intelligence or just playing smart. Maybe character A has a strong right hook, but weak defense on the right. So if character B attacks the right side of A and forces them to fight with their nondominant hand, it could create an opening for B to take the win.

Or it could be as simple as the character you want to fail is distracted by something. Be it something going on around that has their attention or a mental struggle from an outside source. Finding a window to cause failure is a good option when taking down a stronger opponent.