This glitch works when fighting all bosses except the first and the last one. First, when in the stage of a boss, go to the far left of the area. Then, just barely tap the right button so that you just turn around(if you move any except for just turning around, this glitch will not work). Once you have turned around, use the potion of levitation to float. While you are floating in this spot, you will be completely invincible and can attack by using the dagger/axe.