Fight the fight
How about the episode were Fonzie fights a guy at the diner that he believes he found his mother at. He fought a trucker in that episode but is was off camera in the kitchen. Then the trucker comes out and then falls to the ground.
If you're taling about punch for punch, as mentioned above, Fonzie does punch out Howard in the episode where Joanie is dating Marion's ex.
When Fonzie is acting dean at the school, he is at a hearing at the school whether he punched out the studend or not. Through one of the points of view, Fonzie punches out a student. But that was just filmed for that person's perspective on the way it happened.
When Chachi is boxing to impress Fonzie and breaks his hand when someone opens a door where Chachi is shadow boxing, during the fight, the towel is thrown in but Chachi's apponent won't stop, so Fonzie jumps into the ring and knocks the guy out in one punch.
Off camera again, Fonzie went out into the parking lot and cleaned Bernie's clock in the episode where the shrink suggests that Fonzie build birdhouses to work out his agressions.
Frasier: "Don't stare at me Eddie. I'm a humane man, but right now I could kick a kitten through an electric fan!"