There are two chests there. The first one is on a ledge, the other is in the middle of the maze.
I return so I can enter the way I originally entered. Drop Anvils on enemies-- sometimes you can even hit the enemy inside the fire maze.
Use Oswald to float to the left / bottom side, where the ledge is. Thin the gold crystals, and have Oswald boost you up. You can move the fire by pushing the nearest lever twice, so you don't accidentally hit fire.
Don't worry about getting to the middle chest. To your right is a fountain. Spin the crystals to get treasure, then thin the crystals to turn the water yellow, to make you invincible. Take note of where the chest is, then jump in and walk to it. If you didn't previously kill the monster, do so. Open the chest. Turning the lever in the middle will shut off ALL the fire.
Walk to the far ledge where there is an enemy. Turn the handle, so a platform rises, and jump into the Devil statues mouth.
You also get a pin for entering the Angel side and exiting the Devil side.
You can also do it the easy way.... go to the opposite ledge of the ink well and Oswald should be able to toss you up to a ledge. From there, hop onto the top of the beams holding the fire up and walk across them carefully and jump down into the spot with the chest..... or, go up to the Angel side on the spot near the inkwell and bridge, throw Oswald up, grab on to him and float down into the spot with the chest in the maze.