Instructions
Equip one of your characters with the "Haste" materia, and make sure another character has the Cure/All Materia equipped before entering the boss fight.
Cast "Haste" on your party as soon as the battle starts. This will give you an edge against the Materia Keeper because it allows your party to act faster than normal.
Strike the Materia Keeper with physical attacks. Save your MP for healing and support magics such as Haste. You can also attempt to poison the Keeper to give yourself an even greater advantage.
Hit the Keeper with your Limit Break attacks every chance you get. These are your strongest attacks and will hurt the Keeper more than normal physical strikes.
Heal your party immediately after the boss casts "Trine." Trine is a powerful magic spell that hits every member of your party for significant damage. If you do not heal immediately, you may find one of your party members knocked unconscious by the Keeper before you get another chance.