Xion is fought as a boss in Kingdom Hearts 358/2 Days and Kingdom Hearts III. In Kingdom Hearts 358/2 Days she is fought multiple times throughout the game before finally serving as the penultimate boss. In Kingdom Hearts III, she and Saïx team up with each other at the Keyblade Graveyard to fight Sora, Kairi, and Lea.
Kingdom Hearts 358/2 Days
In Mission 75, Roxas is sent to Twilight Town. Xion has left the Organization and is mistaken for the Organization impostor, Riku. Roxas has to track her down and "eliminate" her. Roxas attacks the imposter, but is stunned to realize he is fighting Xion.
She attacks Roxas in Mission 92, which takes place throughout various locations in worlds visited during the events of the game. Using the Organization XIII Devices, Xion is able to absorb Sora's memories from each world in order to replicate his abilities, and to some extent other elements of those worlds. For example, in Halloween Town, Xion uses Sora's Ragnarok and uses a weapon resembling the Chimera arm or The Experiment's arm, while in Agrabah, she uses his Ars Arcanum and has arms and swords resembling Kurt Zisa. Strike Raid is the only one of Sora's Kingdom Hearts special abilities that Xion does not use. Interestingly, the first three areas that Xion warps Roxas to are the locations of that worlds' respective Keyholes. In the case of Twilight Town, the Station Plaza is where Sora opens the world Gate.
During Roxas's battle with Xion, she dons armor which changes a number of times throughout the fight. Her basic form is a bizarrely-shaped, predominantly silver and black suit, with dull red armor near her waist, gold on her boots, and orange armored gloves. Xion's helmet sports two black spikes that jut out horizontally on either side, and a large, red Nobody symbol on the faceplate, part of which forms something of a horn on the top of the helmet. This form is based on Sora's original appearance, with Sora's jacket, pants, and necklace, although Xion's pants are pink and have designs, and her necklace features a Nobody symbol instead of a crown.
In her first form, Xion sports large, skeletal, colorful wings and a huge, orange and red, intricately-shaped sword. In her second form, Xion loses her wings and grows to twice the size of Roxas, exchanging her sword for a large, pincer-like weapon vaguely resembling one of the Chimera's arms. In her third form, Xion transforms into a figure with four huge arms, decorated with elegant gold, purple and red designs. She has large blades instead of hands, and her arms resemble Kurt Zisa's arms. Her main body is slightly larger than normal in this form.
In her final transformation, Xion grows to a titanic size, about the same height as the Train Station tower in Twilight Town. Her armor is now embroidered with numerous spikes and ribbons, and she gains two pink and silver, Keyblade-like weapons with teeth in the shape of the Nobody symbol. Interestingly, this form has a similar design to Sora's appearance in Kingdom Hearts II, now being mainly black and dark grey.
Kingdom Hearts 358/2 Days
Roxas may get stuck fighting the many Heartless in this mission during the search for Xion. Moving further into the Tram Common, Roxas sees Xion on top of a building, and she runs away. Xion does not attack, but only runs and dodge rolls, and teleports to another place in the area after taking several hits. The system for when this mission ends is unclear, as the timing for Xion to go to the secluded area is set at random. At the end of the mission, Xion teleports herself to the secluded place, where she waits for Roxas. Once Roxas reaches Xion, the mission ends, following a few cutscenes.
Be prepared for a multi-stage boss battle. During the fight, Xion transports Roxas to four different locations, each time assuming a different form.
Xion's attacks require good timing and careful maneuvering to avoid. Make good use of Dodge Roll and Glide. Equip Cure magic and/or Potions. Attack magic is effective but not essential.
The first battle with Xion takes place in the Bizarre Room of Wonderland.
The second battle takes place in Halloween Town, at the Manor Ruins.
Xion's third battle takes place in the Lamp Chamber area of Agrabah. In this form, Xion is more graceful and agile, performing flips and spins that make her difficult to hit. Xion's third form is difficult to dodge due to limited dodging room combined with Xion's agility and long reach. Land combos immediately after she stops attacking, and move away before she resumes.
The final battle with Xion takes place in front of the Train Station in Twilight Town. She becomes massive, but remains stationary at one end of the battlefield throughout the fight. In this form, Xion loses both Counter Slash and Barrier, and her regular attacks are slow and telegraphed. However, her attacks can inflict status effects.
Kingdom Hearts III
Xion is fought at the Keyblade Graveyard alongside Saïx. Sora has Kairi and Lea as allies. Throughout the fight, Xion alternates between wielding her Replica Kingdom Key and copying Saïx's berserk state.
Xion's battle skills have effectively increased due to her possessing a fragment of Xehanort's heart. Xion will often teleport occasionally to where Sora or Kairi are standing. Once she reaches her target, she'll use multiple Keyblade thrusts to send the player flying into the air, ending her combo with her slamming them down with her Replica Claymore. She'll also occasionally enter a berserk state like Saïx's but the attack can be blocked by locking onto her. The fight automatically ends once enough damage is done to Xion and Saïx.