This is too hard and I can't tell if I'm supposed to get an upgrade every wave. If I am, there might be a bug where I'm not getting one after each wave.
I think it takes way too long to kill enemies. I feel too weak going into wave 5 where there's too many enemies on the screen and it's difficult to avoid the bats, eyeball bullets, knight swords AND the skulls. The skulls should die wayyyyy sooner than I think. It takes like 3-4 seconds of non-stop shooting to kill 1 at the start.
Overall, I don't feel like I'm rewarded for playing frequently enough (the skill of understanding how enemies attack and roughly when they attack improves over time, but after a certain point it's overwhelmingly against the player).