I got it after the battle with Saix, when I was leveling up my other forms. What I did was go to TWTNW and go to the save point "Twilight's View", then just go to the next room called "Hall of Empty Melodies" and when you get into a fight, go into any drive form, if you don't go into Final, then leave the room and do the trick where you go to a save point (while still in a drive form) and go to the world map, then go back to the save point and repeat. If you go into Anti form, then you will have to raise the bar back up, though.
Also, the room right after "Hall of Empty Melodies" (I forget what its called) that has a shiteload of Nobodies is a great way to level up the final form, since you have to kill nobodies to level it up, just do the same trick mentioned above to keep a full drive gauge.