In certain spots of the world map you'll stumble across a couple of friendly monsters. They are land spirits and each of them will make a request from you. Fufilling their request enables you to be able to use the "Attack" command when you are fighting Ozma, who's unlocked in the Chocobo Garden by inspecting a large rock on the upper right hand corner.

You'll get Garnet's strongest Summon, Ark, from defeating Ozma but it is a very hard battle, so be sure you find all the land spirits:

Squirrel: Around Dali. Ore stone.
Ghost: In front of Treno (but not too far) or around the South Gate. Ore stone.
Ladybug: Around the Black Mage Village. Two Ore stones.
Yeti: Forest outside Madain Sari. Two Ore Stones.
Nymph: Forest near Iifa. Three Ore Stones.
Jabberwock: Forest East of Oeivell. Emerald.
Feather Suckle: Ice Continent, on the Chocobo tracks. Moonstone.
Galda: Forest outside Gizamaluke Cave after climbing rope. Lapis Lazly.
Yan: Bile Island (small island near center of map). It will keep running away until you have found all eight other Land Spirits. Diamond.