Where is the best place to level up final form? ?

If I remember correctly, it's the World that Never Was, cause there you have a lot of nobodies, and you need to defeat lots of nobodies to level up the final form

Okay, do this. Start from the top of the castle, the Altar of Naught (I thinkIi wrote correctly), then get out of there turn into Final Form and defeat the nobodies there. If you still have time, keep going down the castle. If not, return to the Altar of Naught, it will transform you back and refill your drive gauge. If you're in other place, find the nearest save point and return to world map, while still transformed. Lu Xun is right: the best place is The World That Never Was.

I leveled up mine in the World That Never Was castle and leveled up alot so basically what ^ said.

Well ya YWTNW or Mysterious Tower if you'd like variety. Oh and till you get the auto ability Form Boost keep Oath Keeper equipped! That should let you kill a few extra Nobodies!

Just keep going through The World That Never Was. When you reach the top your drive gauge will refilled to MAX. Allowing you to go down through the world again with Final Form. Repeat as much as desired.