Well... The MC is another thing. It's called stupidity. You don't fight someone above your level and hope to win... Its also another thing if they still lose and powers up suddenly. Its a deus ex machina moment but i get your point.
D
Deleted member 84247
It depends on the reason, I think. At least have a good reason that the villain powers up, and if they are going to stick around make sure they are a good/entertaining character.