Nice game! I like the idea! Here's some things - I think - would benefit it:
Make sure to always spawn enemies off-screen (or at least far from the player, currently they can spawn on top of you).
I never really had to use the aim feature, I think the game should've created situations where re-aiming is more practical. For example, you can make hp restroring items that only spawn at the center.
(and of course add the score to the game-over screen)
