As blackmage_nuke said, the only real requirement is to have a character equipped with it and have them stay alive til the end. Relm and Shadow are my favorite characters to use thanks to the Ribbon and Memento Ring protecting from all status afflictions the shield gives. My suggestion would be to teach magic to characters in the Cactuar Desert near Maranda (if you are playing the GBA version, beware of Jumbo Cactuar. If you are worried about leveling too much thanks to the Sand Worms I also suggest going back to the island where Celes begins in the Ruined World as the enemies have ridiculously low health and tend to die on their own since they have a permanent Sap effect on them, if you are fast enough, Locke can steal Megalixers (sp?) from the enemies.