Hey, some suggestions, I think the most important one is to have a crosshair that I can aim with, really hard to know were my mouse is and I only used the other bullets as a guide. Secondly a sound that tells you when you are out of bullets or an animation since you don't have any indication of when that happens unless you are actively watching the bullet count in the bottom right. I also don't really feel the game mechanic that you chose, it would have been more enjoyable for me at least to press right click and make the enemies go back, or if you go this way, rewind the player with his health back since right now the enemies are way to difficult even though you get some health packs on the way.

Hope to see your game developed even more and congrats!