Seymour Flux will use Lance of Atrophy on a party member, applying the Zombie status to them before Mortiorchis casts Full-Life on them, which will kill them. Mortiorchis may cast Full-Life even if Seymour does not cast Zombie.
Mortiorchis also uses Cross Cleave, which deals an average of 2,000 HP damage to the entire party.
If the Mortiorchis's HP drops to zero, it will use Mortibsorption, replenishing its HP from Seymour in the amount of 4,000 HP, 3,000 HP, 2,000 HP, or 1,000 HP.
Attacking Mortiorchis gives an advantage to Magic casters with its lower Magic Defense (except that most magic attacks do considerably more than 1,000 damage), but hampers physical attacks with its higher Defense. Moreover, this strategy inevitably wastes damage (the out-of-game meaning of the term 'overkill'), as attacks will almost always total more than Mortiorchis's current HP. Seymour cannot be overkilled, in the game sense, by an attack on Mortiorchis.