Xion is fought as a boss several times in Kingdom Hearts 358/2 Days. She is fought as the game's penultimate boss.
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.
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 possesses two unique abilities. First, Xion performs a powerful, unblockable counterattack whenever Roxas parries her combo attacks (i.e. using Block). Dodge Xion's attacks, instead of Blocking, to avoid triggering her counterattack.
Second, when her HP is low, Xion may attempt to restore her HP. She floats into the air, channels for a couple seconds, then raises a dark barrier and regains a substantial amount of HP. This ability can be interrupted by attacking Xion while she is channeling.
Xion's attacks require good timing and careful maneuvering to avoid. Make good use of Dodge Roll and Glide. Equipping Cure magic or Potions will aid greatly; attack magic is effective but not essential.
The first battle with Xion takes place in the Bizarre Room of Wonderland.
Her basic combo attack consists of an overhead slash, followed by a forward thrust, and finishing with a spinning overhead slash.
She also utilizes a single-strike attack, visibly charging energy before rushing toward Roxas with a rapid forward thrust.
Xion may enter flight mode: she releases a nondamaging burst of energy to push Roxas away, then floats into the air. She performs an aerial version of Sora's Sonic Blade, attacking with a quick diving thrust. If the attack connects, Xion will perform the attack in succession until the attack misses or is parried. Xion returns to the ground after the attack.
The second battle takes place in Halloween Town, at the Manor Ruins.
Her basic attack consists of an overhead slash, followed by an upward diagonal slash, and finishing with a forward thrust.
Xion may create a vortex to pull Roxas in, charges energy for a number of seconds, then rushes forward with a swift overhead slash. Dodge by Gliding or Air Sliding out of the vortex, or Dodge Rolling with the correct timing.
Xion periodically uses Sora's Ragnarok. She floats into the air, charges energy, then fires a salvo of projectiles which home in on Roxas. Avoid this attack by Gliding, or by running (as long as a level 2 Haste is equipped). Because Ragnarok is a Nil-based attack, equipping a ring with Nil-elemental resistance can aid immensely if dodging Ragnarok's missiles proves difficult. (If Nothing to Fear is equipped, Ragnarok will deal zero damage, though it will still cause Roxas to stagger.)
Much of this stage is often spent dodging Ragnarok. Glide (or run with Haste) around Xion until Ragnarok fizzles out. Watch for Xion's next move. If Xion begins to charge up Ragnarok again, land one or two combos while she is charging, then immediately Glide or run away. Repeat until Xion begins another attack.
Xion's third battle takes place in the Lamp Chamber area of Agrabah. While attacking in this form, Xion performs Sora's Ars Arcanum, infusing her four blades with light. In this form, Xion is more graceful and agile, performing flips and spins that make her more difficult to hit.
Her basic attack consists of two downward diagonal slash attacks with her front blades, finishing with a slow, powerful overhead slash with all four blades.
A more dangerous combo attack involves Xion slashing downwards twice, followed by a jumping upward slash attack and a downward strike, and finishing with a leaping flurry of slashes.
She retains the vortex attack from her second form, charging and attacking with a downward slash with her four blades.
Xion periodically leaps into the air, plunging downward with multiple slashes and producing a small shockwave.
Xion's third form is arguably the most difficult form to dodge, due to limited dodging space and the long range of Xion's attacks. Land combos immediately after she stops attacking, and be wary of staying too close when she resumes.
The final battle with Xion takes place in front of the Train Station in Twilight Town. She attains a massive size, but remains stationary at one end of the battlefield throughout the fight. In this form, Xion loses both her counterattack and her heal ability, and her regular attacks are slow and far more predictable. However, her attacks can now inflict status effects.
Her basic attack consists of a downward diagonal slash with each of her blades, finishing with a diagonal slash attack with both blades.
When Xion crosses her blades, she executes one of two attacks. The first creates a damaging orb of darkness around Roxas; the second fires blasts of light from the tendrils extending from her shoulders.
In addition, Xion may slash four times in succession, each time creating a shockwave of blue energy.
When her HP is low, Xion gains access to two powerful desperation attacks, each creating a vortex in the middle of the battlefield. Which one she uses is dependent on the attack she uses before the vortex appears on the ground. If she shoots the blasts of light before the vortex appears, she puts her Keyblades together, firing a blast of light in the air which causes pillars of light to rain down randomly across the battlefield (an attack based on dual-wielding Roxas's limit break: Magic Hour). Avoid this attack by Gliding and staying as close to Xion as possible, where the attacks are less likely to hit.
If Xion creates the orb of darkness before the vortex appears, she will use Trinity Limit, creating a massive circular pillar of light which engulfs most of the battlefield. Dodge by Gliding up to the corners farthest to the east or west of Xion. (Like Ragnarok, Trinity Limit is a Nil-based attack. Nil-elemental resistance can be equipped to mitigate or prevent damage from this attack.)
Every attack in Xion's arsenal, with the exception of her desperation attacks, can be Blocked. Use Glide to dodge Xion's desperation attacks.
Because Xion's attacks in this stage have a significant delay before each attack, memorizing the visual cues preceding each attack simplifies this fight greatly.