Dun dun DUN! I like it.
That being said, there's a difference between defeating a Warden and killing a Warden. If Harry caught Justin in a position where going full on toe-to-toe was not an option (element of surprise, or Justin in the middle of doing something like a very delicate spell that couldn't just be paused), decades of magical and job experience go out the window, and Harry takes him out easily.
If Harry caught Justin in a position where going full on toe-to-toe was not an option (element of surprise, or Justin in the middle of doing something like a very delicate spell that couldn't just be paused), decades of magical and job experience go out the window, and Harry takes him out easily.
I doubt it - in a flashback in "Welcome to the Jungle" we see Harry fighting Justin, and Justin is casting some kind of electric eel spell at Harry. So I think it really was a straight fight.
I don't think Harry's memories of it could be altered either, because the "perfect memory" thing he had as a ghost in GS probably would have bypassed that - Lea said that his ghost memories were not just stored in his head but remembered by the universe (not exact wording, but it seemed pretty clear IMO that it was "external storage".)
I think Harry's raw power is generally underestimated. He's stronger than Luccio was even pre-bodyswap, and Justin likely was no great shakes.