An enjoyable little shooter!
With the amount of enemies that spawn, I think a machine-gun (constant bullets) would make this game crazy fun! If not that, I would allow the player to spam LMB to shoot lots of bullets. The delay between bullets here felt a little too slow.
Other than that, perhaps make the enemies single-hit? There are so many of them that shooting them twice becomes un-manageable.
Great game! Looking forward to seeing more.