Master Xehanort appears as a boss in Kingdom Hearts Birth by Sleep. In Terra's story, he is fought twice in a row. The first fight is against both Master Xehanort and Vanitas, but the fight is ended prematurely, and the second fight is the penultimate fight of Terra's story: a one-on-one duel between Terra and Master Xehanort.
Master Xehanort also serves as the final boss of Kingdom Hearts III, where he is fought by Sora and the party in Scala ad Caelum. He is fought twice in a row. The first fight is against him in his armored form and the second fight is against him while he wields the legendary χ-blade.
Kingdom Hearts Birth by Sleep
Terra will fight both Master Xehanort and Vanitas two-on-one. Vanitas should be the primary target as Xehanort will mostly dodge and keep a distance from Terra. Xehanort isn't very difficult and after sustaining minor damage, the battle will end.
The second battle is the true battle against Master Xehanort. Like the previous battle, Xehanort will keep a distance, but now has access to wide-ranged magic spells. All of his attacks, including magic-based ones, can be dodged with the right timing. Cure commands and Shotlocks ease this battle significantly.
Kingdom Hearts III
Sora should have several Elixirs equipped, with Curaga set as a shortcut. Stocking a Kupo Coin is also helpful. The Ultima Weapon will be the most efficient Keyblade to have equipped, but Hero's Origin, Shooting Star, Crystal Snow, Hunny Spout, and Wheel of Fate are also useful for their Formchanges.
After the Xehanort Replicas are defeated, Master Xehanort absorbs them and dons his new Keyblade Armor, then reshapes the plaza into a closed chamber to initiate the beginning phase. Keeping track of Xehanort's position can be difficult due to his speed and the size of the arena. Airstepping can be used to stay close to Xehanort, but can sometimes result in accidentally leaping to one of the flagpoles dotting the arena. Speedy or homing attacks such as Flare Force, Double Arrowguns, Hunny Blasters, Highwind, and Blizzard Blades will be useful for dealing damage, while the defensive Counter Shield can be used for its Wrathful Fist reprisal.
After getting his health down to a certain point,[?] Xehanort will move the battle underwater, initiating the middle phase. As formchanges cannot be used underwater, Sora will not have access to Rage Form.
Finally, after reducing his health even lower,[?] Xehanort will move the battle to the stormy skies above the city, initiating the closing phase. Sora will be alone during this phase, meaning that Donald and Goofy cannot heal or support him. Sora should focus on dodging Xehanort's attacks and healing when necessary.
Armed with the legendary χ-blade, Xehanort has an arsenal of powerful attacks at his disposal. Sora should focus on defense, as "Tataki Otoshi" will prevent him from landing attacks easily. Reprisals such Counter Blast and Wrathful Flurry are useful for injecting damage, as well as speedy Formchanges like High Wind, Blizzard Blades, Twin Yo-Yos, and Nano Arms, or homing techniques like Flare Force.
After losing seven and a half health bars, Xehanort will initiate the second phase of the battle by summoning Kingdom Hearts via "Ascension", then immediately launch into "Dark Shooting". He will continue employing his earlier techniques, albeit enhanced via "Ascension".
After four more of his health bars are depleted, Xehanort will use the power of Kingdom Hearts to steal Sora's light, forcing him into a version of Rage Form that cannot terminate or use Riskcharge. In order to return to normal, Sora must attack Xehanort to force him to drop orbs of light[?] which both fill the Situation Gauge and restore HP. During this portion, Xehanort cannot be directly damaged, but will continue to use his enhanced attacks. After filling the Situation Gauge, Sora can use "Return to the Light" to attack Xehanort and revert back to normal. Xehanort will respond with "XIII Combo", then continue attacking as normal. If enough time passes[?] without advancing to the final phase, Xehanort will steal Sora's light again, forcing Sora to retrieve it once more.
Once Xehanort is down to 1 HP, Sora can use "Last Ray". However, Xehanort quickly counters and retaliates with a beam of darkness that seemingly kills Sora. Sora can then use "Trinity", ending the fight.
Notes and references