Invincibility when fighting a boss:

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.

 

More Gems:

When you get to level two, use the Red Key to enter through the Red Door. Go all the way up and you'll find a gem block. Hit a maximum of five gems out of it and get them. Then go left and fall all the way to the bottom. Go back up and repeat this procedure to get more and more gems.