When it is in Final Mix+ you have to beat Roxas first.
You get his Weapon "Acrossing Two" equip it and start fighting against the heartless there.
You need 5 Drive bars at least and just start Driving when you're in battle!!!!
While Driving with "Acrossing Two"
the Chance of getting Anti-Form is so Incredibly high, that Final Form could be activated too.
If you Drive in Anti-Form run outside and go here again...
The point is Driving while in Battle! Outside Battle, you never will gain Final or Anti Form!!!
If you choose Valor, Wisdom or Limit Form to Drive, then it's normal that only one (or in Limit none) of the Characters vanish.
But if they vanish both, even by choosing Valor Wisdom or Limit, the Chance gets high for Anti or Final Form!
After beating Roxas is a chance Rate of 25% to get Final Form... if you Drive into Anti, the Chance Rate boosts up to 50% and if you get Anti AGAIN you have a chance of 100% of getting Final instead of Anti! With "Acrossing Two" the Chance of getting Anti/Final is also boosted!
BEATING ROXAS in TWTNW!!!!
5 Drive Bars
Not more! No Leveling etc. just this!