Easy Spheres!!
The easiest way to get power, mana,etc. Spheres? Go to one of the first places you ever went to, such as the Djose Highroad or that Kilika Island place, and fight the easiest battles in the world, each harboring basically all Overkills. Before you know it you'll have tons of spheres! (you'll need the airship)
If you need more help, we've got more Final Fantasy X cheats and also check out all of the answers for this game
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