I beat The Bugs Bunny Birthday Blowout (NES). I don’t remember if I beat it as a kid, but the final boss did look familiar. It took me maybe 15 lives for the last level, though it was only the last two tries that I figured out definitively how to hurt the boss. I had around 50 lives left just in case.
I think the game took around 3 and a half hours. Most of the times when I died, coming back through, my timing got much better at everything I had played through already, and I could usually speed back to where I was. The last level was the only one I really struggled with the timing on, though I did eventually get to the point where I could get through most of the damage points without getting hurt.