I really like the idea ^^ It could become a very fun game.
There's a few issue you can improve on:
The buttons in the menu selection aren't working properly. My guess is that the hit box of select level is bigger than the sprite and covers the other buttons.
The aiming doesn't follow the mouse correctly. I know it's very difficult to get it right but it's frustrating. I can try to help you understand how to fix it if you want.
The hitbox of the ennemies are too big. In a game, you always want your ennemies hitbox to be smaller than the model it's affixed to. Otherwise the collision won't feel fair to the player.
When I hit play the game, I got thrown to level 4 for some reason.
Add maybe an arrow to indicate where the ennemi is going to go after seperating
Hope it was helpfull ^^