Using the mouse, the arrow keys/WASD and space at the same time is kinda awkward. maybe better to auto shoot or auto aim so you either do not have to use the space bar or the mouse.
Also having a sprite for the enemy would be nice, even it would have been a simple sprite, the transparant green blob was sometimes a bit diffiult to see against the background.
I really like these kinds of game. These games can be so addicting, I think your game was close to being that