Black Waltz has 71k and Sealion has 92k. Both are weak to Fire and can be slowed, so that is your main strategy.
Start the fight by using Magic Break and Slow on both of them, since their spells hurt. After that, focus fire Black Waltz, since he is able to revive the Sealion if it dies before him. You could Mental Break him + Firaga or Armor Break him + Fira Strike. Whatever you want to do.
Once he is dead, focus fire on the Sealion. When it gets pretty low, it will cast 'Tsunami', a powerful water AOE magical attack, so you need to keep it magic broken and, if possible, protect yourself with Tyro's Sentinel.