From what I've gathered, the chances of achieving Final Form is greater if you form change in The World That Never Was. The Memory Skyscraper area is a good place to try. Pick a form, change, and if it doesn’t work, just head back to the Brink of Despair, leave the world and come right back and try again. Lather, rinse, repeat until you get Final Form. If anti-form shows up again, just use the Coliseum trick to max out your drive gauge and try again.

By the by, is it just me or did anti-form seem more powerful in The World That Never Was?