To be able to go against this secret boss, first you must have completed the game, which means you must defeat the end boss. Since now you can save after defeating the game's end boss. The other objective would be to defeat the 5 Organization XIII members in their "Absent Silhouette" battles. Their battles can be accessed in different parts of the game worlds. After both of those are done, a portal will be open in Disney Castle "Hall of the Corner Stone". The portal leads to the secret boss.