This is an interesting take on this type of game. I am not very good a bullet hells so I just kind of found my self hugging the outside wall 90% of the time waiting for my chance to get in and deal the big baddie some damage. The game wasn't too difficult for the most part. Losing minions was very painful but just making a massive circle around everything kind of let me get around that mechanic most of the time.
I enjoyed the art but the sound effects leave a bit to be desired. Nothing felt really good to me and sometimes I didn't even know I was getting hit.
Overall, great job. I could see this turning into something interesting with more polish if you continue to develop it more going forward.