It seems Sora get's psi powers in final form I call him Perfect Sora.
Final Form (The Easy Way)
About the easiest way to get Final Form is to "fight" Roxas in The World That Never Was and then go to The Underworld & fight in the Goddess of Fate Cup. Since you get Final at a random time, when you go into Anti Form by chance, you should fight Hades in the final round. It is impossible to turn into Anti Form when you fight Hades (as well as Pete & Barbosa), so your best chance is to keep going into Valor or Wisdom form (since they cost less drive gauges) hoping that you would turn into Anti Form. But since you can't you would either sucsessfully change form or get Final form.