This game should be called "Extreme Bullet Dodge"
in the later levels there are just so many things going on that you don't focus on killing the enemies anymore but dodging the many bullets that are scattered around the screen. It becomes extremely difficult to continue because there are just too many enemies on the screen intent on killing you, great idea but it's a bit too hard. I've also found that ironically, the boss fights are actually much easier than the lead-up to it, but yes, even with the upgrades the game gets frustratingly hard.