I couldn't finish 'em. Looks like this's gonna get complicated.
This battle is somewhat difficult. At first, this boss makes use of some elemental Ninjutsus for about 700 damage and his physical attack will damage more than 1000 to a front-row member. When his HP gets lower, his stats are boosted. His Ninjutsu will do about 40% more and he will use attack such as Assassinate or Great Gale to either instantly kill or drop one's HP to critical status.
Two Dancers with Cure Waltz or a White Mage with Curaja should be enough to keep the characters healthy. His attacks are strong, especially when his stats get boosted so healing consecutively is recommended. A Summoner with Tidal Wave, a Black Mage with some strong spells a Monk or a Dark Knight (preferably with Back Liner) along with Jinnai can damage him well. It is a good idea to heal from time to time as this boss changes to Mind Parasite immediately after his HP runs out.