Anticlimax Boss: During the Auto-Scrolling Level that takes place before the last level, you will confront The Three Mischievous Fairies. Now the bosses before have put up some sort of fight and offered a bit of a challenge. For this battle, the bosses will simply fly at you. All you need to do is attack each one once. Very anticlimactic for the penultimate boss.
It's interesting to note that the music for this battle is the same as the first boss's. If that isn't an indicator that this will be easy, nothing is.
That One Boss: Nue Houjuu. Unlike the other bosses you fought that can easily be brought down in three hits, this boss is defeated in 15 hits. Before the battle, you would have picked up a UFO powerup, which you will need to retain by picking up more. What makes this boss difficult is the fact that she throws other powerups at you, and if they touch Marisa, it's game over.