The dark energy coursed through the caverns, and collected in the deepest part, where in formed a giant red sea turtle with a shell of cooled magma. It also had the ability to swim through lava, of which the battle arena is completely made up of except for a small platform in the center.
Throughout most of the battle, Moltagus will swim fast around the platform you stand on. You must homing attack into the shell 8 times while he is beached (when his sliding attack fails) to destroy the shell. This initiates the second phase, where you can actually damage him by hitting his soft body 10 times.
Moltagus' primary attack is to belly-slide over the platform when he isn't swimming in circles around the platform. Sometimes this attack fails, and he gets stuck on the platform. This is when you can attack him, but you only have 2 seconds before he slides off the platform into the magma. He also spits 3 volleys of 6 fireballs in your direction, so you must avoid these.