Go through the door to enter the second green room.  Find the fence and climb up it, then drop a bomb down on the diamond switch and climb up the fence once the pillar of fire is gone (similar to the last green room).  Go through the door up there to enter the large donut-shaped room.  In here you'll find a small yellow switch and a long circular stairway.  You have to step on the switch and run up the stairway before the time runs out to open the treasure chest at the top.  Of course this is pretty hard because the stairway is right on the edge of a huge pit.  If you happen to fall you'll be back where that Goron was that you couldn't save before inside the pillar in the middle of the big fire maze chamber.  You'll have to make your way back up and try again.  Once you open the treasure chest you'll get the Megaton Hammer.

Now make your way down the narrow stairway to a block with a face on it.  Hit it with the hammer and fall down the resulting hole.  Now smash the statue with the hammer (this is harder than it sounds...try holding forward or sideways while swinging the hammer to hit it correctly).  Now in the next room kill those annoying bats and then hit the small face tile with your hammer.  Grab a crate, go down the stairs, and put the crate on the blue switch.  Go through the door.  In the next room hit the face square with your hammer, drop down, then hit the rusted switch with your hammer.  Go in the door.   Play the Song of Time to move the blue block down.  Use it to jump to the other side and then hit the switch with your hammer.  Go get the key from the now-freed Goron's cage.  Then climb back up to the door you came through using the blue block.

Back outside, hop over to another face block and hit it with your hammer.  Make sure you have enough life to handle it and then drop down the hole.  If you're low on life there's a fairy in a pot on the right side of this room once you're down there.  Use the hookshot to get to it.  Anyway, exit out of this room and you're back at the entrance.  Go down the stairs and to your left is a statue thing. Destroy it with the hammer and go through the door behind it.  Kill all the baddies in this room and go on to the next one.  In here the floor tiles will raise up and try to attack you so watch out.  If the Shield-eater gets your shield and tunic kill it quickly and you'll get them back.  Go on to the next room and you'll face the second dancing fire man mini-boss.  Kill him the same way as the first one.  Go through the next door, smash the rusted switch with the hammer, and nab the boss key. Now go out the doorway ahead of you and you're back at the entrance.  Go up the stairs and through the left door, hop across to the boss door and go through.  Now you'll face the level boss.