Mortibody regularly casts Desperado, relieving all positive statuses. Seymour Natus will use Break on a character, and Mortibody will follow up with Shattering Claw, shattering the petrified character. The Mortibody will later cast Cura on Seymour Natus, which can be reflected back onto the party.
If the Mortibody is defeated, it will use Mortibsorption, draining HP from Seymour Natus to regenerate to its next amount of HP (first, 4,000, then 3,000, to 2,000, and finally down to absorbing 1,000 HP each time). Mortibody will keep doing this, even if it kills Seymour.