The Final Episode is unlocked upon beating all three characters' stories, as well as collecting all Xehanort Reports. Make sure to have saves for all three characters with all Xehanort Reports collected to receive a notification stating that the Final Episode has been unlocked. You will then be prompted to make a new save, where the final part of the trios' story can be accessed. However, due to the circumstances Terra and Ventus have found themselves in, only Aqua is playable. After the scene, visit Radiant Garden to battle the game's true final boss.
Aqua travels to Radiant Garden in search of Terra only to be faced with the possessed version of her old friend last faced in Terra's scenario at the Keyblade Graveyard. Terra-Xehanort will use a moveset very similar to that battle, but since Aqua's fighting style is different than Terra's, a slightly different strategy should be followed. Aqua has access to both D-Links and Command Styles, as well as Shotlocks. The player should make full use of these special abilities whenever possible.
It is recommended that the player Cartwheel away from the boss and use the Mine Square ability. Terra-Xehanort will most likely Dark Haze dash in the player's direction surrounded in darkness as he did in his battle with the Lingering Will, but he will be damaged by the mines, his attack interrupted and opening him to more damage. However just like before, there is a chance he'll most likely dash around any attack that would be landed on him in the open, with trap-based attacks being no exception if Terra-Xehanort happens to dash somewhere else towards a possible blind-spot. Because of this, commands like the aforementioned Mine Square should be used more to limit and control his movement instead of outright trying to damage him.
Once Terra-Xehanort is back on his feet, however, the player should follow a hit-and-run strategy, bombarding the boss with magic rather than physical attacks and either running or Cartwheeling away immediately afterwards. The player can also use Barrier to defend against Dark Volley, also using a Barrier and Counter Blast combo to prevent damage from most attacks and still deal damage. It is recommended to use Renewal Barrier as it can be a big help with restoring HP from successfully blocking attacks in case any Cure spell is unavailable and also make sure to pack at least two of them in your Command List to help make healing easier, preferably Cura or Curaga, but also be ready to move quickly after healing to avoid damage if possible if you are too close to Terra-Xehanort. Terra-Xehanort will summon rock pillars around him if the player is at close range, so use caution when attempting a combo that involves linking defense to a physical attack that only works at close range. As Terra-Xehanort's HP starts to dwindle, he will gain access to the Dark Impulse Command Style. He also may use Curaga to restore health, thus increasing the length of the battle.
In terms of fighting Terra-Xehanort as Aqua, the player should favor magic, as her physical attacks are relatively weak, but be careful as he can dodge your magic attacks. While it is quite possible to go through the entire battle without using a D-Link, Command Style, or Shotlock, they will certainly make the battle easier, mostly Shotlock attacks since they can be used at a distance. Also use counters if you need to fill up your Focus gage (as long as he is not using Dark Volley), as close range attacks are too risky to use if this fight, and should not be used too often. So long as the player uses patience when fighting this boss, he should not be a challenge.
Terra-Xehanort's great rage causes his shadowy other to spawn behind him. He then uses this creature to do much of the fighting during his second battle with Aqua for him. The guardian will use many powerful attacks, and Terra-Xehanort can still attack with the fast-paced physical combo from previous battles, so use caution. As with the last battle, Aqua can use Command Styles, Shotlocks, and D-Links, and having the Renewal Barrier to heal from successful blocking will also be a big help.
In early stages of the battle, the guardian will vanish from behind Terra-Xehanort and begin to strike from underneath Aqua. This can be avoided with timed Cartwheels or parried using Block and Counter Blast; however, Aqua should time the Counter Blast correctly by using it after one strike has been blocked and done to take advantage of its invincibility frames to avoid the incoming next strike. If the guardian's attack lands, it will grab Aqua and slam her into the ground to deal heavy damage. Once it returns to Terra-Xehanort, strike the boss with the most powerful attacks possible and continue with a hit-and-run strategy. Terra-Xehanort may avoid Magic or Shotlock attacks by going from side to side when he isn't using an attack. The guardian may sometimes attempt to shoot twin energy balls at Aqua. Use Barrier to block these projectiles, as they can blind players. Cartwheel should be used to dodge when the guardian sends dual waves of energy in Aqua's direction, as this attack is unblockable. If Terra-Xehanort ever attempts dashing towards Aqua, to use his Ars Solum combo, Block him and knock him away with a Barrier and Counter Blast combo. This combo can also be used if the guardian ever attempts to punch Aqua with its mighty fist. The boss and his symbiote will repeat these attacks until much of their HP is depleted.
Once enough HP has been depleted from Terra-Xehanort, the guardian will try to approach to grab Aqua. If it succeeds, immediately break away by mashing , while Terra-Xehanort slowly approaches her as well, and if he is close enough, he will use his Ars Solum combo on Aqua. Either way after that, Aqua will be transported to a realm of light, and Terra will stand in the middle of the area, stunned, while the guardian continues to attack by firing twin energy spheres and dark punches. Cartwheel over to Terra's location, and enter the pool of light surrounding him by pressing to activate a special Deck Command called "Dual Limit". At the right moment, press to team up with Terra to strike the guardian with light. This attack will deal three quarters of a bar of health if you get an "Excellent", and half if "Good". Aqua is then transported back to the main battlefield and the attack process beginning from square one. For this reason, it may be beneficial if you let the guardian grab you to use this Dual Limit and deal some extra damage, instead of blocking it. It is important to know that if one strikes Terra rather than using the finishing command, then Aqua will be transported to the normal battlefield again, as well.
This incarnation of Terra-Xehanort is not difficult to defeat. So long as the player makes full use of the Cartwheel ability to dodge attacks, they should take very little damage. The player must remember to keep at a constant hit-and-run strategy, utilizing physical attacks rather than magic, and only using the Barrier and Counter Blast combo when a physical attack from the opponent must be defended against. The player can now heal with Curaga and without the fear of Terra-Xehanort healing as well, meaning this battle can go relatively quickly. The player should also try their best to activate the Dual-Limit command in the light realm. If this strategy is followed, then defeating Terra-Xehanort will be child's play. Terra-Xehanort's fall will also represent the completion of the Final Episode, unlocking more rewards in the game.
If playing the Final Mix version, move on to the Secret Episode.