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):
|Larxene||Port Royal - Isla de la Muerte|
|Lexaeus||Twilight Town(2nd Time) - The Sandlot|
|Marluxia||Beast's Castle(2nd Time) - Beast's Room|
|Vexen||Agrabah(2nd Time) - Merchant's Shop|
|Zexion||Underworld - Cave of the Dead|
Thanks to Revolution Reader Blutobot!