I've yet to beat this game, but I've played through some of the higher levels. It seems to be alot harder if you try to play through slow. If you approach the game like a speed run challenge and just try to blaze through a stage non-stop, I find it to get a little easier. It just takes memorization and a heck of a lot of precision (and sometimes luck when facing Bowser: shooting fire, jumping, and throwing hammer crap while a rotating fireball thingie spins next to him).
Also, powerups are pretty much useless. In a couple castles with mushrooms, I found I was dying because of my height. What a bastard of a game, punishing you for trying to gain an advantage!