This dungeon is really simple, but there is a Boss Battle at the end. Collect all the treasure, then head for the southeast corner to fight the boss.
Use any Brave Strikes you may have available, then follow that with your shared skills.
If you use Karate Chop, make sure Kodan uses it.
He gets a "strength up" bonus on this skill.
If Orochi's not dead after that, have Alex and Toril use unisons and Lita and Kodan use their attack skills.
Orochi should go down after just a few turns.
Once you have the Optalir Grass, you're done here, so head on to Hushet Ruins - Other Entrance.