Once you complete the game with a third character and have obtained all of Xehanort's Reports you can create a new save file called, "Final Episode." If you load this save file you will start as Aqua and after a series of events you will receive the Brightcrest Keyblade . You have free reign over this file just like the others with the exception of the Radiant Garden where the ultimate battle will ensue. It is recommended to obtain the best items in the game via the Mirage Arena for the Lightbloom Shotlock and Ultima Keyblade/Royal Radiance. When you are prepped and ready head to the Radiant Garden.
BOSS - Terra/Xehanort
Before you start this battle make sure Aqua has Once More and Second Chance because without them this fight will be extremely difficult. A maxed out Dark Screen wouldn't hurt either to mitigate some damage. Equip numerous healing commands as well as Magic Hour and Time Splicer because they work wonders. Using Barrier is the key to winning this battle because it can deflect the Shotlock bullets back at Terra for decent damage. When Terra attacks with a combo, a simple block and Counter Blast serves it purpose. Terra will be sliding around the battlefield, when he stops fire up a Magic Hour or Time Splicer. If you are going to Shotlock don't bother trying to get a max lock because Terra will dodge most of them anyway. Once you defeat Terra the presence within him rises for a second round of action.
Round 2 is easier than the first but can still prove difficult. Terra will use a new attack, firing twin blades from long range. Use this opportunity to make up for the lack of Shotlocks in the first round. The timing should be as such; start to lock when his arms turn blue and release right as the waves reach you to become invulnerable. Again, Magic Hour and Time Splicer are great commands for when this attack is being used or you are playing defense. Periodically Terra will pull Aqua into an alternate dimension and throw dark orbs at her. Cartwheel diagonally towards Terra and don't bother attacking. Instead wait for the trigger command, Square then X, when you are close to Terra to perform "Dual Limit" and deal massive damage.