[Hydra]

For the easy way, use Eject. He's pretty suspectible to it.

For one thing, the lower his HP, the lower his strength. So, beat the crap out of it. Some of his attacks only hurt Black/White mages, so be sure to switch every now and then. Start out the battle with a White Mage dress sphere. Cast reflect on it, because it's going to use curative magic if you hurt it enough. Either that, or go in with all the girls as Berserkers and the abilty "Evade and Counter". Then use Items to heal if they should ever connect. If they connect often, your level might be too low. Either that, or have the accessory 'Catnip' and Trigger Happy Level 3. 9,999 each and every shot. O.o

Edit:Forget the Cat Nip thing. You get that after beating Dark Elemental in that dungeon.