There are several options for Final Form. Unfortunately, you are stuck at killing 4 until you can get more bars. Then you can kill 5!

WtNW: begin at Proof of Existence, and either go into Addled Impasse or Havoc's Divide. Once you begin to level up, you can try expanding it to the section of the Hall of Empty Melodies right before Proof of Existence.

Alternatively, you can try Twilight's Passage (the stairwell) a few screens after Twilight's View and a few screens before Proof of Existence. This is not recommended because of the non-proximity of a Save Point, especially for lower level Final Forms.

Twilight Town: Yen Sid's Tower. There are a great amount of Nobodies in the Tower itself, and since a Save Point is nearby the entire time, and the Nobodies are easier (slightly), it should be less of a problem.

If you run out of Drive Bars, and revert back, you can also use those Drive Reciveries that you never thought of using (or you should not have used them), or go into Olympus Coliseum and refill you guage by entering and exiting a tournament. Unfortunately, this is time-consuming, so don't run out of those bars.

Don't forget to equip all Drive Boosts and all possible Drive Boost Keyblades if you feel Final form doesn't have enough Bars yet.