There is limited ammo count, but I don't see any way how to get more ammo.
And when hit by enemies, there is no health system/ game over condition. It's just about getting as much score as possible with limited ammo count. Then you can just bump into enemies and that's it.
But anyway, the concept with bigger delay when enemy hit you is interesting. Maybe you could add a time limit and in that limit, kill as many as possible while avoiding them to get the low delay of shooting? And once per some time drop something, what will decrease that delay.
But I totally get it, 3 hours is not that much. Visually game itself looks nice with some post-processing effects.