Arright rapid fire. Add a sound button in the options so that you can adjust the music.
If you could, maybe add some enemies or secondary attacks to the bosses where it fits since some of them are pretty simplistic and easy to game.
Boss 5 gave me the most trouble but it a was a decent challenge.
Boss 3 you could maybe remove the indicator for his skill recharge and just make him unpredictable?
Edit:Yeah i tried hard mode and it IS challenging but still beatable.Bosses 4 and 5 kick my ass. The bullets you fire sometimes dissapear completely, when i am near the screens edge.Could you just add more intense music for Boss 5 on hard mode? So it shows hes the final challenge because he is.Damn that bullet spam.
Overall,its not a bad game,good job.