man, this is a pretty solid entry! I really liked the boomerang mechanic. It adds some depth for the game. Forces the player to get a good grip on the spacing for the attack to be effective.
I really liked the bosses. Each took me a few tries to beat. That ninja on the third level was really though!
Got to level 4 and then decided to stop as it got uncomfortable for me to play (I'm using a 60% keyboard, so I have to keep the fn button pressed to access the arrow keys...) but I will try again another day for sure :)