Zackwell is a spirit of the Death City Mayfil given charge over the souls there. The party encounters him when he wrongfully tampers with the soul of Lavitz. The battle is fought in conjunction with Lavitz's Spirit.
Hit Points: 5500
Hit Points: 8000
While fighting Lavitz's Spirit, you shouldn't attack him until he turns around and his back is exposed. Simply guard until Dart gets a choice of what to do. Each time, choose "Talk to him," and the spirit governing Lavitz's body-soul conflicts with Lavitz's soul inside, and he turns around, allowing you to attack the strange object attached to him. Do so, only attacking him when his back is turned, and eventually the object on the back turns into the demon Zackwell.
Attacking him with Light-based Attack Items or Miranda's Dragoon Magic is probably wise. Use powerful Additions and other magic attacks as well. Zackwell has some very powerful attacks, some that cause status ailments. It is noteworthy that at one point he morphs himself into some strange creature that is invincible to attack. At this point, simply guard or heal until he returns to normal. This battle has potential to be difficult.
When the battle is over, Dart performs his finishing move.