First, for the upcoming battle, you’ll want Water-absorbing or Reflect-inherent equipment on. I’d use the Reed Clock and/or Tortoise Shield for my party, personally. They aren’t good for non-Imps’ stats (and we kinda need non-Imps), but they’ll become a god in the face of Leviathan’s (another god) Water attacks.
Try battling Sprinters on the northeastern continent for Reed Cloaks. Take your pick. Lotsa healing or lotsa pain. Anyways, head to either Nikeah or South Figaro. Save outside, then go into town and opt to ride the ship to the other port. You’ll find the legendary sea serpent on the way. Don’t you feel special?
Remember, I did suggest that you bring Reed Cloaks or Tortoise Shields for everyone to prevent their being effectively drowned. I’d also bring along Fenrir and Golem (if possible).
Leviathan normally will use a basic attack, Aqua Breath, or El Nino (super Aqua Breath). When hit with non-magic, he’ll use Entangle (sets Slow). When hit with Magic/Lore, he’ll hit with El Nino. Tsunami is also used every now and then, sometimes twice.
Since you have the Water-absorbing equipment, 60% of this boss’s attack repertoire is good for you. Given that his Magic Defense is weaker, I’d opt to just cast whatever you want; that El Nino is very good for you in this state. To counteract the odd physical, cast Fenrir and Golem.
That’s pretty much it. I suggest having Terra or Celes along for the help in healing from physicals – once they eventually hit, they will hurt.
After the battle, enjoy the Leviathan Magicite that hits the deck.