I'm getting strong Noitu Love 2 vibes from this game, and I think it's a well above average boss rush game, though not the most original game, but it's still very enjoyable. Though having said that there are some things I'd like to comment on.
*I didn't find much use for the other weapons than the basic shooter, as it did roughly the same amount of damage as your other weapons yet it was the safest one to use.
*The levels felt like more like formalities than actual good content, and I would thought that it would have been awesome if they served as primers for the boss, with enemies having similar attack patterns as the boss so the level would serve as a teaching tool for the player.
*The way the last boss was setup felt a bit odd, it's cool that he had several forms, but the fact that it had checkpoints made the battle feel a bit cheap, like I was supposed to die a certain amounts of times before beating it. Is there some kind of reward for doing a no death run or was this a random design choice?