Just continue to play through the storyline and come back to Atlantica at the end. You will be able to beat it quite quickly.
Good luck!
Also:
The Limit Form is only for Kingdom Hearts 2 Final Mix+. You can get back to the main form by allowing the Limit Form to run out of time or just pressing revert.
Here are all the forms:
1. Valor Form
2. Wisdom Form
3. Master Form
4. Final Form
5. Anti Form
6. Limit Form
How to get the different forms:
1. Valor Form: gotten after visiting Yen Sid and talking to the fairies right after taking the train from Twilight Town to Yen Sid's castle.
2. Wisdom Form: gotten after beating Disney Castle and the Timeless River world.
3. Master Form: gotten after seeing King Mickey in Hollow Bastion
4. Final Form: gotten randomly after seeing (playing in Kingdom Hearts 2 Final Mix+) the Roxas vs. Sora battle. Just switch to a drive form and you will randomly get it.
5. Anti Form: gotten randomly when tranforming into a Drive Mode.
6. Limit Form: gotten (only in Final Mix+) while in Twilight Town late in the game from what I understand.
Allies sacrificed when using the form:
1. Valor Form: Goofy
2. Wisdom Form: Donald
3. Master Form: Both party members
4. Final Form: Both party members
5. Antiform: Both party members
6. Limit Form: no one
How to level the different forms:
1. Valor Form: hit an enemy once to gain 1 experience point, easiest place to do this is in the first area with an almost unlimited amount of heartless in The World That Never Was
2. Wisdom Form: kill one heartless to gain 1 experience point, easiest place to do this is at Pride Rock with the Magnega trick on all of the flying heartless at the top or in the first area in The World That Never Was
3. Master Form: get a small drive orb to gain 1 experience point, gain a large drive orb to gain 3 experience points, the best place in my opinion to level this is in Beast's Castle after new heartless are placed there. Otherwise, try gaining levels for this form in the Land Of Dragons.
4. Final Form: kill 1 nobody to gain 1 experience point, the best place to do this is either in The World That Never Was in the Nobody's Castle or in Yen Sid's tower by going to Twilight Town and taking the train there later in the game.
5. Anti Form: Can't level this form
6. Limit Form: Defeat an enemy with a limit to gain 1 experience point, best done, well anywhere from what I know.