To avoid getting hurt by the lava, just have a Geomancer in your party. You won't fall down holes either if you have one.

Anyway, all the way to the right on the second floor of the room, there's a set of buttons in the shape of skulls. Most of these skulls are traps, but one of them will teleport you to Carbuncle, and it's the skull that's at the top. Just find your way over there by trial and error since you have Geomancer on and you won't fall down.