[zmztdvzj] The hero believably becoming powerful

The hero starts a normal person like one of us, grows stronger, then fights the villain and wins.  Which stories tell "growing stronger" believably?

Harry Potter should have been portrayed as exceeding all his teachers and the most powerful good wizards of both the past and present by a large margin before taking on Voldemort, the most powerful evil wizard ever.

I am far from conviced that Luke Skywalker had become one of the most powerful Jedi ever by the time he faced and defeated Darth Vader, portrayed as one of the most powerful Sith ever.