How do you unlock the Final Form in Kingdom Hearts 2

You know how Twilight Town disappears from the World Map during the game? Once it reappears, start playing it until you complete the Memory Skyscraper part of The World That Never Was (the scene where Riku fights Roxas). Then go to Olympus Coliseum and enter the Cerberus Cup (any level, but Level 1 is recommended).
Once the level starts, switch to Master Form. If this doesn't switch you to Final Form instead, just bring up the Start menu and select Retry to start the level over. Keep repeating this process and you'll eventually switch to Final Form. After this happens you can use Final Form whenever you want, wherever you want.

NOTE: Where other forms can only be leveled up by fighting Heartless, Final Form can only be leveled up by fighting Nobodies.

you can also get Final Form from SIAX boss in the world that never was but you have to be at least lvl 60 or 74 i always got it at lvl 74 but only once at lvl 60 or the first boss fight in the world that never was you can win it
Best way s to go to Olympus Coliseum, go into the cerberus cup. as soon as you enter, change into master form. If it doesn't work, reset the cup and do it again. Eventually you'll unlock final form, and there you go. Have fun, it took me a while.