Don't know if you still need help, but here goes...
in the big lava pit room with the bridge, go off to the left until you reach a platform that rises in the air. Use Din's Fire to light the torch in the little alcove, then with an arrow, light the torch that's in front of the barred door to your right.
It's impossible to get up there as it is, so traverse over the lava to the other side of the room. Use the hookshot to grab onto the unlit torch that you did before, and light it with Din's Fire. Hookshot targets will show up all over the room, and then get up to the door, free the Goron, and get the Boss Key.