I liked the idea and I think there are some very interesting ways to expand this game, but I had a couple of issues with it.
The main one is that the enemies spawn faster than the time it takes to kill them, so the better strategy is to just try to get past them.
However you collide with them and they block your path, and since the gun is very weak it creates this frustrating situation where they all gang up on you, don't let you pass them and kill you very quickly,
I only got to the third level, but I think that there is potential here with a few tweaks!