The easiest way I've found to trigger Final Form is right after Sora creates the bridge to the Castle in the World that Never Was. Until you move on to the next room, that room will serve as a safe zone, with your party breaking up, thus saving your points. Then you backtrack to the previous room (Memory's Skyscraper), get in combat, trigger a Form, and move back if you don't get Final. As far as I can tell, Final Form will only trigger if you Form change in combat.