Cool game and i really like the spritework, that space dragon looks sick : D
I really like that you made it a 3 hit instead of 1 hit kill, it helps better learning the attack patterns.
Even though there's a health bar to indicate the bosse's hp i feel it would help to have a bit more visual feedback on their sprite as well to tell that the shots do in fact deal damage.
Other than that that was fun and i like the idea of this being a boss rush only thing.
Good job to the team :)