Just before the last battle, Myria will transform herself into the Goddess of Destruction, with about 20,000 HP. Defeat her by transforming Ryu into Myrmidon (Frost + Trance + Force), then choose Aura. This will allow more than 1,000 damage to be done on each turn. Do this repeatedly until she has been defeated. Be ready to use your healing items and assign one or two of your party members to replenish Ryu's AP.