This happened during practice against the third boss.

Somehow, I was able to preserve the projectile I grabbed from the boss, and I continue hitting him with it indefinitively! I don't know how to reproduce it yet. I'll have to analyze it later, but if it can be reproduce during a run, it could save so much time!

I'm don't know every details of this glitch, but since I was able to replicate it during 2 runs, and I trained it a little bit more, I'm gonna add more details. When I'm gonna be sure how it works, I'll probably do a post in the "Guides" section.

It seems like it's a frame perfect trick. When you're facing a direction (Left or right), you have to try to grab a projectile with the hand in your back (the left hand if you're facing right, or the right hand if you're facing left). On the same frame, you have to grab the projectile, and being hit by it. After that, I'll be able to "punch" the boss infinitely with the projectile (or whatever it's in your hand. The sprite can be glitchy).

This trick is possible for sure against the third boss. The best way to perform it, is when the boss is moving on a horizontal line, because it's way easier to time it (It's still possible if it moves diagonally, but it's hard to time this frame perfect trick in this case, since the projectiles trajectories can be hard to predict). When you achieve it, don't worry about entering the boss hitbox. You'll probably have the time to destroy its hit points (which are in the number of 8 ) before the invinsibility frames ends.

Concerning the second boss, since it can also shoot projectiles, the trick might be possible as well, but I never tried it. So it's still to be tested.

Here is an example where I was able to perform this glitch in a run: