I have made a list of how hard the boss's are:
1. Bau(mutant dog). pretty easy
2. Death face: the most easy boss in the game
3. Tape worm: short battle if you find a quicker way to defeat him (with powerups I think)
4. Eyeball in mouth that shoots stars? (I have no idea what it is): with rockets it's very easy
5. Shark: get a spread gun or homing rocket powerup to make him easy.
6. Pink big body thingy with creapy looking head? (no idea what this is either): err very easy
7. King crab: well you don't have much room but i would rate him sorta easy
8. The dreaded cilia monster (big worm looking thing): im not counting the part were you have to go past him so he will be easy when you destroy that gut gun thingy on the wall
9. Space men: very easy
10. robot boss: get those powerups or you will be dead in less than 5 secs or will be very hard to defeat so overal hard
11. 5 eyeballs: the hardest boss in the game
12. Core monster: Get rockets or the ghosts will kill you. He is easy with powerups