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Terra-Xehanort is a boss in Kingdom Hearts Birth by Sleep, serving as the final boss for Terra, who fights in the form of the Lingering Will, and then for Aqua in the game's Final Episode. He also appears as a boss in Kingdom Hearts III, and is fought alongside Vanitas in a section of the Twist of Isolation in the Skein of Severance within the Keyblade Graveyard.
Attacks
Combo: Terra's standard three hit combo.
Counter Hammer: Blocks then unleashes a large shockwave on the ground.
Dark Slide: Terra's slide and sonic impact, although similar to Dark Haze.
Ars Solum: A multi-hit combo which is faster then Terra's; one hit ensures that the rest will also hit.
Quake: Three rock pillars rise around Terra-Xehanort.
Dark Volley: Shoots dark energy bolts which home in on the Lingering Will or Aqua. The weaker Terra-Xehanort is, the more shots are fired.
Meteor: Casts a spell which brings a giant meteor crashing down over the Lingering Will or Aqua.
Dark Impulse: Goes into Terra's Dark Impulse and unleashes the three hit combo then plunges into the ground for a dark hand attack from underground.
Curaga: Casts Curaga to heal Terra-Xehanort.
Meteor(2): Terra-Xehanort teleports to the middle of the arena and casts a spell. Rocks then rise out of the ground and stay aloft for a while before bombarding the Lingering Will.
Ultima Cannon: A last ditch attack where Terra-Xehanort enlarges the Keyblade, charges, and then fires.
Strategy
Terra's Scenario
For the final boss fight of Terra's scenario, the player takes control of the Lingering Will in order to battle Terra-Xehanort. It will have access to any abilities Terra had equipped before the battle, and its Keyblade is preset to the Ends of the Earth. D-Links cannot be used during this battle, but Command Styles and Shotlocks are still available. However, Shotlocks like Dark Volley won't work as Terra-Xehanort will immediately start to dash. Therefore, it's best to use rush Shotlocks like Lightning Ray.
In early stages of the battle, Terra-Xehanort will immediately go on the offense, usually using Dark Haze, dashing around the battlefield, surrounded in darkness. The player's best defense against this tactic is to use Block and immediately follow with a Counter Hammer. This will do great amounts of damage to Terra-Xehanort, causing him to warp away. When the boss reappears, he may attempt a fast-paced physical attack similar to Terra's Ars Solum ability. This can be Blocked against, opening the boss to damage from Counter Hammer.
He may also utilize the Shotlock Dark Volley, but the dark projectiles he fires can be deflected back at him to deal damage with Block. Should one attempt a physical attack of their own against Terra-Xehanort, be advised that the best place to strike is from behind. This is mainly because Terra-Xehanort also has access to the Block and Counter Hammer combo.
As Terra-Xehanort's HP starts to dwindle, his true power is unleashed. He may use a variation of the Meteor spell, causing several large rocks to erupt from the ground and bombard the player. While this attack can potentially deal heavy damage, it can be defended against with a well-timed Block. Don't try to go for Terra-Xehanort at this time as he will escape by teleporting. He may also summon a large meteor if the player is at close range.
Usually when the player is at close range, Terra-Xehanort may cause large pillars of rock to shoot out from the ground around him, in a similar fashion to the Quake spell.
He will continue to use the Darkness-based attacks from earlier stages of the battle, which can be defended against in the same ways mentioned above. Terra-Xehanort may also use the Dark Impulse Command Style, which involves him landing several hits with his Keyblade and then striking the player with an immense hand of dark energy (which can break through Block, so dodging is advisable).
Terra-Xehanort will then vanish into the ground, visible only as a pool of darkness on the ground. This pool will travel underneath the player, during which time the boss will attempt another dark hand attack.
This attack requires precise timing to dodge, the player needing to wait for the pool of darkness to travel beneath them and then using the Slide ability, which can also be used to avoid many of Terra-Xehanort's attacks or get the Lingering Will out of attack range. As Terra-Xehanort's HP lowers even further, he will chain the close-range rock pillar attack with the Meteor spell.
He lastly will have access to the deadliest attack in his arsenal, Terra's Ultima Cannon Shotlock. Similar to when the Lingering Will is fought as a boss in Kingdom Hearts II Final Mix, Terra-Xehanort will enlarge the Keyblade and charge its power, firing a single, large orb of energy to hit the player. This attack will deal a great amount of damage, and cannot be defended against with Block like in Kingdom Hearts II Final Mix. however, the player can use a well timed aerial slam and knock the attack back, damaging terra-xehanort. Or the player should simply do whatever they can to stay as far away from the boss as possible. Also, beware when Terra-Xehanort's HP is low, as he starts teleporting more frequently.
In short, this battle against Terra-Xehanort is all about an equal balance of defense and offense. Defensive abilities should be used which then link to an offensive attack, such as the Block and Counter Hammer combo.
This way, most damage is avoided while the player can still deal heavy damage to the boss. One should also be careful when executing the Curaga spell. While this will fully restore the player's HP, Terra-Xehanort can heal as well, increasing the length of the battle. So long as the correct strategies are followed and the player uses patience, Terra-Xehanort will fall and Terra's scenario will have been completed.
Final Episode
First Battle
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 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, opening him to more damage. 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. 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. He has 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. 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. So long as the player uses patience when fighting this boss, he should not be a challenge.
Second Battle
Terra-Xehanort's great rage causes a dark symbiote known as the Guardian 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.
In early stages of the battle, the Guardian will vanish from behind Terra-Xehanort and begin to strike from underneath Aqua. Chain consecutive Cartwheels together to avoid this attack, or the Guardian will grab Aqua and slam her into the ground to deal heavy damage. The Guardian will follow Aqua as she Cartwheels, but the attack will always have a high chance of missing. It can also be guarded against with Barrier. Once it returns to Terra-Xehanort, strike the boss with the most powerful magic possible and continue with a hit-and-run strategy.
The Guardian may sometimes attempt to shoot twin energy balls at Aqua. Use Barrier to block these projectiles, as they can blind players and render Shotlocks useless. Cartwheel should be used to dodge when the Guardian sends dual waves of energy in Aqua's direction. If Terra-Xehanort ever attempts a physical attack, 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. If the Guardian ever latches onto Aqua, immediately break away by pressing 
. 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, Aqua will be transported to a realm of light. Terra will stand in the middle of the area, immobile and non-offensive, while the Guardian continues to attack by firing twin energy spheres. 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 one bar of health if the player gets an "Excellent", and half if they get a "Good". Aqua is then transported back to the main battlefield and the attack process begins from square one. This also will happen after the guardian grabs Aqua, followed by Terra successfully hitting the player with a combo. For this reason, it may be beneficial if the player lets the guardian grab them, if they have enough health to survive the onslaught. 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 magic rather than physical attacks, 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 finishing 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.