This is an interesting battle, but not a particularly difficult one. The battle will start off with three Mantas on the enemy's front row, and Mantalas in the back. Each enemy has a basic physical attack that usually deals between 100 and 150 damage, with a small chance to Paralyze their target.
However, Mantalas has one extra trick to make the battle interesting; whenever it is damaged, it will counter with Bizarre Howl. This skill will call between 1 and 3 Mantas to replace any Mantas the party managed to kill off (but the number of active Mantas will never exceed 3) before Mantalas itself leaps off the ship - the only way to bring it back to battle is to slay all of the active Mantas.
Since Mantalas is going to make itself untargetable the moment you hit it, you'll want to make sure every hit counts. For starters, take out the three Mantas to reduce the enemy GC to zero; if you have some spare turns, buff your party - you'll want to up your Attack/Magic Attack and possibly your defense.
Once Mantalas's defensive line is gone, hit it with Grounda to deal over 1,000 damage. After it uses Bizarre Howl and jumps overboard, you'll have to be careful when selecting your actions - you don't want to waste your one strike at the Mantalas with something weak. One tactic is to have Seth and Kaim both attack the first Manta, then select Grounda with Jansen, Cooke and Sarah (it won't matter what target you select.) If done properly, Kaim and Seth will act first, taking down the first Manta, two of the mages will take down the second and third Mantas, and the Mantalas will return just in time to be hit by the third Grounda. If you're using Mack, have him cast Minda on Ming, Sarah or Jansen; if you're using Cooke, have her on healing support or equip her with accessories that enable her to cast Black Magic and join the Ground(a) assault.
Depending on your party's damage output, you might not hit the Mantalas on the same turn it reappears. If this happens, let your strongest caster (likely Sarah) use Grounda while everyone else Defends or heals. As long as you plan your attacks, this fight won't be too long.
Note: if you take too long defeating the Mantas after Mantalas jumps off the ship, it will simply leave the fight and end the battle automatically.