Access the ES/AU Battle:
To do this, you first have to beat the game. Then, beat the five Organization XII members (see below). After that, head to Disney Castle and look for a new portal called "Hall of the Cornerstone". There you'll find the secret boss battle.
Where to find Absent Silhouette battles (Organization XII):
||Port Royal - Isla de la Muerte
||Twilight Town(2nd Time) - The Sandlot
||Beast's Castle(2nd Time) - Beast's Room
||Agrabah(2nd Time) - Merchant's Shop
||Underworld - Cave of the Dead
Thanks to Revolution Reader Blutobot!