Xaldin is fought as a boss in Kingdom Hearts II and Kingdom Hearts Union χ. In Kingdom Hearts II, he attacks Sora on the bridge outside Beast's Castle, and is the second member of Organization XIII Sora faces in the game.
Kingdom Hearts II
As the battle begins, Xaldin will cloak himself in Aero magic, causing all attacks to deal minimal damage to him. He does this periodically throughout the battle.
Initially, Xaldin will either thrust his lances directly at Sora or gather them and swing it around him. However, if Sora sidesteps the attack or uses Guard, he can deflect Xaldin's attacks and use the Reaction Command "Learn" to replace the Attack command with "Jump". Depending on how many times is pressed to use the command, Sora will stock that many jumps up to a limit of nine. The Jump command pierces the Aero and also allows Sora to start an aerial combo on Xaldin immediately after. Xaldin will also use the Jump command himself, jumping up out of range and striking Sora with powerful blows with his lances a few times before striking Sora by jumping down with all of his lances. The initial shockwaves can be jumped over and the final Jump can be guarded.
Later in the battle, Xaldin begins attacking faster and teleporting short distances and although Sora can still use the Jump command, Xaldin moves so fast that Jump is likely to miss. His attacks increase in range and power, and he will whirl his lances around to attack rapidly. Since he will now smash Sora with Aero while he attacks, Guard will prove ineffective and it is better to use Reflect to deflect his lances.
When low on health, Xaldin will summon an aura and become invincible as the screen turns dark. He attacks quickly with his lances, which deal massive damage if hit, before teleporting next to Sora and connecting his lances, leaping atop them. The lances take on the form of a dragon, which launches a blast of wind along the length of the bridge. Sora can Quick Run or Dodge Roll through the wind, or simply cast Reflect. If the wind is launched from one side of the bridge, it will not reach the other end. Xaldin will use this attack more as he loses health.
Xaldin is a very difficult opponent, especially late in the battle when he becomes invincible and uses his rapid combos. Fortunately, King Mickey may be available when Sora is defeated, giving him a second chance. If Mickey appears, Xaldin's dragon attack can be used to an advantage, as it gives time to use Mickey's D-Charge without interruption. In the shortcut menu, Sora should have Reflect to deflect Xaldin's attacks, Potions and Cure to heal, and Ethers to replenish MP. Be sure to use the Jump command whenever possible.
Kingdom Hearts II Final Mix
Xaldin returns as a Data Rematch battle and is much more powerful. His health has greatly increased, but the strategy to defeat him remains largely the same. Of note is that his Aero shield now reduces all incoming damage to a mere one point, and that coming in contact with it deals massive damage to Sora.
To prepare for the fight, stock up on Elixirs, have the Cure spell in the shortcut menu, and equip the Ultima Weapon. Guardian Soul is also an asset, as the Jump command will prove handy. Maximize the Growth abilities granted by Drive Forms, as they can be put to great use during the battle. For example, whenever Xaldin starts his lance shower attack, a good strategy is to Glide around the area while the pressing to get any possible Jump commands, then cast Reflect on the last attack to inflict massive damage. This protects Sora from damage and from danger of being defeated before healing. Even if this technique only yields two or three Jump commands, Sora can simply use Jump to dispel Xaldin's Aero spell and follow up with an attack combo.
When Xaldin attempts to ram Sora with his Aero rush, use Aerial Dodge and land close to him. Doing so will keep Sora safe from Xaldin's attack, while creating an opportunity to collect Jump commands. Jumping or Dodge Rolling can also be used, and still allow Sora to learn Jump. Xaldin will continue pursuing Sora unless attacked, so take advantage of this pattern to heal if necessary by Gliding away.
When his HP is low, Xaldin will gain access to his dark aura mode. By this point in the game, Sora should have learned Once More, greatly reducing the threat of this attack. The attack can be avoided by Gliding away, as Xaldin remains stationary. His Tornado attack can be avoided simply by casting Reflect. The best time to attack Xaldin with Jump is when he ends his Aero rush, when he stops for a brief moment, or when he attacks with his Lances.
Mickey and Beast will not assist Sora in this fight.