Ultimately, you just gotta cheese him.
Your levels should be fine, but get Aqua up to around 60 if you’re struggling too much. I’d say Aqua is the easiest to beat him with since her dodge ability is the best of the three.
As Xickin said, use Fire/Thunder Surge. Don’t just use them to attack him, use them defensively too; you get decent iframes from the surge attacks. This is especially important for Terra since his dodge sucks. Push comes to shove, Shotlocks can give you some brief invincibility to allow for your Cures to recharge if you need them, but it's a bit trickier since waiting for them to lock on leaves you vulnerable.
If he’s coming at with an attack, always go defensive. Using a Surge to protect yourself whilst doing no/little damage is better than leaving yourself vulnerable just to do a bit more damage. I especially remember his light-whip grab-attack thing being especially devastating, but if you get the timing down right, you can use a Surge to block the attack, then as he falls to the ground again, damage him with another.
My setup for him is always 6x Thunder Surge and two Cure spells next to each other. Set your shortcut to Cure so you can instantly snap to it for when you need it. Cure loads faster than Cura/Curaga and you don’t need a full HP bar, just enough so Second Chance/Once More activate when you need them.
Don’t look at his HP bar. Just focus on what he’s doing ‘cause he’s a slippery little sneak. Been a while since I've fought him so don't really recall any of his telegraphing or anything, but just keep at it