Cool, I like bullet heck types of games so I had a lot of fun and the art was nice and simple, and your particles are gorgeous!
For improvement I would recommend evaluating the feel of the gun at low levels, as the flying enemies are somewhat fast and the starting stats make them really hard to fight until you get upgrades (where the player's skill should improve). Perhaps just give the bullets a more generous base speed.
On the post I got here from you guys said you were pretty young--this game is way better than anything I ever did around that age! Good job!