Use wind elemental magic and weapons. Buy the Gladius ( I think its the gladius) and get the most powerful wind magic available.

Before the fight, buff everyone with whatever buffs you have, like bubble, protect, etc. I like to equip all three characters with reflect rings and spam my most powerful Aero spell on my party, hitting elder wyrm 3 times for massive damage. Add in the gladius wielder for extra damage and make one character a dedicated healer and you shouldn't have too many problems.