The hub was perfect. Good, foreboding music, nice atmosphere, real boss rush hub-y. Nice work on that. Now, the bosses:
- DK Bro: Slightly more challenging than your average Birdo. Basic, but functional. I dig the boss design, though (something about being a massive DK fanboy

)
- Bomb Bro: Again, basic but functional.
- Wart Bro: I feel like I'm repeating myself, this is, like the other two, average all around. He IS a bit easier than most Warts, so he fits well as a third boss.
- Piranha Experiment: Good idea, but destroying the Venus Piranhas makes this fight pathetically easy. Maybe if you added reskinned Rinkas instead?
- King Mole: Made really easy with that ledge. If you made it just one block smaller, the fight would be somewhat challenging.
- Koopa Bros: Again, the ledge makes this far too easy. The arena is also slightly too small.
- Wart: Cool idea, but reskinning Larry's shell to be bigger than Larry himself makes it impossible to jump on him without getting hit. A spinjump works just fine, though. You could have gone all the way and added jumphurt to him. Also, when Wart goes all Tasmanian Devil on you, he moves FAR too fast, which is made impossible to avoid by the arena, which is MUCH too small.
- Pom Pom: Basically King Mole almost done right. She can get back up on the higher ground, but the slope makes killing her by sliding possible, which makes her too easy.
- Bowser?: Not much to say here

- Bowser: This dragged on for far longer than it should have. The first phase has an awkwardly small arena, which made it slightly more difficult. The second phase was next to impossible with how absurdly MASSIVE those fireballs were. Speaking of the fireballs, they had no right-facing sprites.
All in all, you played it quite safe, and it paid off with some pretty passable boss battles. There's still room for improvement, though. Also, you really put some more thought into the backdrops for the bosses, adding a nice sense of variety as well. The music was also well-chosen.