I like this game a lot! You've got a really neat concept, nice boss variety with the different attack patterns, and a great natural difficulty curve of trying to not get shot by your previous rounds. The art and character design is cool as well.
I think the difficulty select might be bugged? I tried a round of each difficulty at first and saw pretty much the same attack patterns. However when I on a whim went back to try the game again I totally restarted the page, went to the highest difficulty and saw the bigger projectiles and the large bullet waves. I'm wondering if the difficulty variable doesn't get cleared or something unless you start a new session, or maybe my session was just unlucky.
Awesome job!