On this floor, head immediately south to find Genji Armor. From there, cross the bridge and enter the water. You'll have to work your way to the southern stairs out of the water, then head east to find the Fang of Earth and Hecatoncheir. He is a simple boss with a powerful physical attack, so having your front line use shields is recommended. You don't actually need to use Mystic Knights in this fight, but it's unlikely you'll have anything better available. Keep pounding away at him. He is a bit weaker against magic than physical attacks, but either will suffice, especially if you use Spells like Haste.
Hopefully you have someone who can cast Teleport (or brought an Ottershroom), or else you're walking out.
Hopefully you have someone who can cast Teleport (or brought an Ottershroom), or else you're walking out.