I killed the boss, it's cool you have one, the scope of the game is not huge but a final boss is something. The game controls well, the bullet speed feels right and the difficulty is fine. I like life and fuel being the same stat and decreasing automatically so you need to take risk but sometimes taking too much risk ends up not being a good bet. The difficulty felt right, I won on my second try but ended the game less than one hit away from death.
One thing I want to mention because we dealt with something similar when making our game (I ended up not allowing the game to play the same sound more than once in a tiny period), I think 3 bullets ended up making the same sound than one but just louder sounds weird to me, but maybe it's just me.
Thanks for sharing.