It's not the hardest by a long shot, but I never did beat it without cheating. I can definitely see myself beating it if I exploited boss weaknesses and patterns and such. The only truly cheap area in the game is that damn water level with the bombs.
Yep, I've beaten it before. The dam/water part is a bit tricky for the first few attempts, but it doesn't take that long to get it down and move on to more hair-pulling areas (like the Technodrome ). Not sure if I'd have the patience to go back through that game again and relearn all of those patterns enemy and boss, though.
A good friend of mine played that game like a million times when he was a kid, he can still beat it pretty easily. I however never beat it, I never got past the water level.
The only way I've ever beaten it with cheats, and then it took a few tries. If only you knew all the times I had invincibility on and fell into the fire near the very end. Awesome game though.