Well the game is fairly easy, I played through it twice just to get all the ammo packs for the medal and i didnt die once. I feel you have the right idea with the depleting energy but you just give way too much batteries to make it a challenge and if you either amp up the energy loss or greatly decrease the energy packs it could make it more challenging. I also thought the lack of collision detection for the bullets was pretty "amateurish" no offense, it just seems like something that shouldnt be in a game.
TL;DR Great idea just poor level design gives the game a lack of challenge.
