Definitely a pretty damn interesting game overall. I never would've thought to make an endless runner work as a boss rush game, but I'd say you definitely did it pretty well. Although I do have some issues
First off, I love the simplicity of the game in general. The controls are tight as well. Love that holding up or down has the cat walk upright lol
The presentation of the game as a whole is damn great, both audio and visual-wise. At first I loved the spin effect on the sides of the bell. However, after a while I felt like it started to become something of a detriment since it was a little hard to tell when some objects were actually close enough to me yet.
The projectiles as a whole were pretty well balanced, although I feel like the ghosts could've used relatively smaller hitboxes than what they currently have. Or atleast they could've been spread out a little more. I also think that the cacti could've been a little more noticeable or stood out more since sometimes I couldn't even see them ( Probably just a skill issue though ). I do also wish there was a little more feedback when you do damage to the boss. I wasn't actually sure what ringing the bell did. Sometimes it seemed like it caused damage to the boss, other times it seemed like it just changed the type of attack thrown at you.
I'd say the rat bosses on the bell were done pretty well, however I think the next boss suffered from quite a few issues. Primarily in terms of readability. I'm not really a fan of both the boss and the background having blue as a primary colour. It kinda caused the book and the pickups that you need to damage the boss to all blend into one another.
I also think that the blue projectiles should've honestly looked differently since they looked a little too much like the pickups you needed to collect. Especially since they also kinda blended into everything else like I mentioned above. I do think the damage feedback is better in this boss than with the bell though.
At the end of the day, despite my issues, this was still a damn fun game. Great work on this one