So first of all the aiming and shooting is really good;

But the enemy really doesn't make sense at first; he just floats around and does nothing;

I am not really sure how i'd improve it, but I would probably make him shoot since the start making it a scramble to find the ammo;

Also I'd make the enemy die in one hit and the ammo not respawn;

That being said it's just what i'd do; I am not sure that this would be the best experience that you might have intended :)