I really like it. Evey stage of boss is really different and requires different tactics. Thats nice. Some constructive opinions:
For most stages of the game best tactic is just run in circles. Maybe some movement prediction would be nice?
Pushing gun is so powerful that it often kills boss. I found that not using it and just spam bosses around is more convenient. I don't think it's idea you had in mind.
I really like your aesthetics.