Boss Rush Tips:
Shellbreaker: Just stay bottom left, you'll be safe
Feet: Stay between both eyes always, spam Up Right and Up Left
Cube: Always kill block first. Watch out for reverse patterns
Moonsnail 1: Hold fire down and you can't be hit - just dodge the orbs.
Moonsnail 2: Be on the reverse side when he does the crescent attack. Try and stay half way up the screen when he does the spinning attack so you can go either up or down easily.
I'm not sure why people are hating on the game, I thought it was good for a short game. I was stuck overthinking level 7 and after that didn't really have trouble.