In the world of ruin (near the east side)there should be a tower surrounded by mountains. Go into the tower via airship and put the best people at magic in your party (because you're only allowed to use magic in the tower) keep going up the stairs of the tower until you get to the top (after about 1 hour). Go into the room and get the gem box (X-magic) out of the chest and before leaving equip it to your strongest magic warrior (mine was terra). It is also a great idea to give your character with the most hp a hp raising relic/equip(I had a red cap that raised hp by 20%). Exit the room and wait for the scene to finish, you will now battle a real tough boss as he keeps using an ability that changes his element. Cast shell on the character with the most health. Now, randomly cast different Fire 3/Ice 3/Bolt 3 spells. Once his hp reaches 0 he will counter-attack with ULTIMA, your character with the most hp and shell should still be alive and you win the battle! Gem box is a great relic as you can use 2 spells in one turn. Works well with a gold hairpin relic as it halfs mp consumes so you can use 2 spells for the price of 1!!

Note: you do not have to go all the way back down the tower, it will automatically do that when you defeat the boss.