Walk down the path, killing the Nobodies and collecting a Blue Orb along the way. At the end is a room with a T. Rex skeleton in it. This one doesn't spit fireballs like the one in Mission 17, only belches fire like a flamethrower, and must be defeated with good old-fashioned elbow grease. Any Devil Arm works well, but it is impervious to gunfire. After the "dragon" is dead, two Frosts will spawn (Shadows on Easy). Use Ifrit to kill them. There is an unconventional Red Orb Crystal between the dragon's legs as well. Break that to net some extra orbs.
Once everything is dead, moving platforms will spawn. You must jump across these to reach the Bangle of Time.