To get a Keystone, you must first get a warrior spirit(s), which is obtained by opening 6 chests in a row. After that, once you complete a world you must take the top route to go to the alternate path, then you must go through that level with every time you die you have to use a warrior spirit. I found the easiest one to do this on was the world 2 alternate path. (I got three warrior spirits in the first world, getting all the chests and never losing equipment). So you will have three chances to get through the whole level (There are two checkpoints). If you make it to the room right before the boss, you will find the keystone. Good Luck.