Some bosses were too hard for me, especially the one who randomly changes my pieces when I rotate them. Angle Angel was easy, despite being a rare boss (iirc).
I wish the tutorial didn't start with dialogue. I immediately skipped it and started playing blindly. Only after the guy who randomly transforms my pieces beat me did I go back to the tutorial and realize it wasn't actually boring.
I think starting with action right away (instead of the map) would be better. I know it's not easy to change, but maybe you could even completely embed the tutorial into the main game.
Bug (?): After finishing the tutorial, it sends me straight to the main menu. I used the sledgehammer and thought I'd killed the boss, but a second later I was back at the main menu.
A heads-up: Windows Defender flags it as "Trojan:Win32/Wacatac.B!ml". I looked it up, and others have reported it as a false positive. I don't know if there's anything you can do about it though.