This is how I beat him, my characters are all at level 36.
I made three of my characters White Mages, and one a Black Mage. (I've usually been playing with two characters on melee, and two on casters, for the entire game.) The Black Mage and one White Mage I equipped with the ability Dual Cast, and the other two white mages I just gave 30% HP bonus, because it was the only "melee" ability that would help them during the fight.
Before the fight, I cast float on all my party members, to avoid the big 3k damage earthquake that the turtle does before he dies.
Then, it's pretty easy. Just cast protect on all your party members at the beginning of the fight, and have the Black Mage Dual Cast Blizzaga. Also, I found that keeping up the Blink spell on everyone decreased the need to heal. It has three charges, so the upkeep was pretty simple. You don't even have to cast Esuna or anything to get rid of the poison and blind debuffs that he puts on your party, because you can heal through it, and you're not meleeing anyway.
After that, sit back, and heal as needed.
I ended up using an elixir on my Black Mage, as he ran out of mana two rounds before the fight was over.
So, yeah! This may be a long fight, but it's hard to fail with this strat.
If you had more than two caster characters, you could equip them with better abilities, like more black magic, or time magic, because the fight basicly takes care of itself, after the initial protect buff. I just did it this way because I've been playing with a half and half system.
Anyway. I hope that this helps.