Gi Nattak
Gi Nattak is a specter that haunts a cavern system under Cosmo Canyon called the Cave of the Gi. It is accompanied by two Soul Fires which possess the abilit to possess your characters for a time, occasionally causing fire damage to them. They are also able to heal themselves by casting fire on themselves.
To help make this battle easier you should try to equip items that protect against fire based damage. Nattak's main weakness is holy and oddly enough this boss can be easily killed with a Phoenix Down or Megalixer. If you go the proper route and actually fight Nattak be aware that earth, gravity, and water. Any attacks with those elements will not work on it.
Keep plummling it with normal attacks and magic that are not the above mentioned elements. Keep the party's hp stable as well. It is best to just ignore the Soul Fires unless they become a real nuisance. Use limit breaks and the become available and this boss fight sohuld be over pretty quick.
Or you could just go the cheap-o route and use the Megalixer from the previous area to kill this boss in one hit.
You get a Phoenix Down and a Wiser Staff for defeating this ghost from the past.