Wow, this was a lot harder than I expected it to be. I don't know if it's having to deal with the flying controls or just the boss' attack patterns, but I had some real trouble with D. Snut and Ms Toad; I just couldn't figure out how to avoid them, especially Ms Toad.
I found the invincibility window a tad too short as well, cause some bosses can hit you twice with the same attack, which feels a little unfair.
Difficulty aside, though, this was a really solid game. Excellent work!