Interesting "last stand" idea.
I think it'd feel better if the player had to hold/press the mouse button to fire so they can time their shots more easily. You could also consider increasing the projectile speed (and at the same time the enemy spawn rate/move speed/health/etc).
Might also be cool to make it a sort of planetary defense thing where instead of the enemies coming from the top, they come from every direction and you're in the center.