Saïx is fought as the third-to-last boss in Kingdom Hearts II and Kingdom Hearts 358/2 Days. In Kingdom Hearts II he is the last of the Organization XIII members to fall before Sora faces Xemnas, attacking Sora near the top of the Castle That Never Was, in his chamber overlooked by Kingdom Hearts. In Kingdom Hearts 358/2 Days he attacks Roxas when the Nobody tries to leave the Organization.
 Kingdom Hearts II
Throughout this battle, Saïx's attack pattern is determined by a "Berserk Gauge" in the top-left corner of the screen. When the battle begins, the gauge will be full, and Saïx will rise into the air and enter his "Berserk" state. The gauge will begin slowly emptying, and once empty, he will revert to a calmer attack pattern, wherein the gauge begins filling again.
While in Berserk mode, Saïx attacks very quickly with powerful attacks, slamming his claymore into the ground to unleash large shockwaves of energy. He also throws his claymores at Sora, unleashing streams of energy when they impact. While in this state, Saïx's attacks have a large range and high power. In addition, he is invulnerable to normal attacks, and extremely resistant to magic attacks. However, the player can pick up his thrown claymores with the Reaction Command "Berserk", and then use the Command "Eclipse" to slam Saïx into the air, and finally "Magna Storm" to attack him rapidly. If "Magna Storm" connects with Saïx, his Berserk gauge immediately empties, and he reverts to his "normal" state.
While in this state, he walks slowly and attacks with slow, but powerful combos. At this time he is more vulnerable to damage, but he can still defend himself, so it's best to use Guard when he attacks, or to deflect his attack with your own, to make him drop his claymore. This will cause him to cease attacking for a while, as well as give you an opportunity to use Reaction Commands to attack with the dropped claymore. At times, Saïx will stand still and raise his arms to hasten the speed the Berserk Gauge fills. As Saïx grows weak, the Berserk Gauge fills at a much faster rate.
As long as the player uses Reaction Commands to revert Saïx's transformation as soon as possible, he is not an overly difficult opponent. If you find yourself in a tight spot, avoid Saïx's berserk attacks by gliding or dashing away, healing yourself as necessary. Be cautious if you use Drive Forms, as Anti Form is more likely to activate. Using Final Form is a big help if you have it, while Saïx is in his normal state.
 Kingdom Hearts 358/2 Days
Saïx is the third-to-last boss of the game. His main tactic is his berserk mode, which he starts almost at the exact start of the fight. Don't move towards Saix until he activates his berserk state, as being near him when he activates will deal damage. In this state, his Claymore has its signature extended white edges. He himself has an aura of blue light surrounding him.
He has a diverse range of attacks and combos at his expense. He has a few close range combos in which he swings his Claymore around. If you get caught by one attack, you may get drawn into the rest of the attacks he does for that combo. He has five types of combos.
The first one is his most common attack, where he swings the Claymore for about 2–3 seconds ending in a spin. There is about a 2-3 second gap until his next combo, so get a few quality strikes in before retreating to avoid his next move.
The second pattern is where he also swings his Claymore around, but there are distinct differences between this and the first, which you may notice. After about 4 seconds, he will swing the Claymore down in a straight, vertical slash. That's your hint to run away into the air, because he will follow up that slash with another of the same slashes, but it will produce a blue energy wave in its direction. Immediately afterward, he will jump in the air and throw down his Claymore, which will strike down anything in its vertical path (including the air) and produce an 360 shock wave which will hit anything on the ground that is near it, even if your blocking. This will open up him to a few strikes from you. However, you can't use his Claymore to hit him and the damage you do is the same as usual in his berserk defense. After 2–3 seconds, he will summon his Claymore to him and start attacking again.
The third is a simple two strike move move with one swing and a spin. It takes about 1 second for him to execute this and the traditional 2-3 second gap after.
The fourth is a mix of the second and third with the third coming first. Right where he should throw the Claymore, he starts the second combo. After this, again comes the 2-3 second gap.
The fifth is a truly berserk state where he moves in circles, swinging wildly.
Another of his attacks is where he teleports to the top of one of the towers in the room. After another 2–3 seconds, he will leap over you and drop down to smash you. On occasion, if you move right after he leaps, he will stop near the ground and teleport above you to smash you again. Either way, he will immediately follow up with a combo.
After a while, when you deal enough damage, his aura will "break" off, he will slide backwards, and he will become inanimate for a short while. That's where you nail him with everything you've got: magic, critical hits, combos, anything. After that short while, he will start to defend himself with blocks, jump strikes, and 3 hit combos. It's important you still prioritize defense, and Saïx is still a danger even when he's not in his Berserk Mode, as he still will not stagger from attacks. Soon after, he will restart his aura.
Some tips: 1.His attacks and Aura start will randomly put Silence on you, making you unable to use magic.
2.His Aura provides extra defense against both magic and attacks.
3.Attack whenever possible: Set up Blizzagas to hit him when he's attacking; Use Thundaga while he's on the tower.
4.If you use Thundaga and break his Aura while he's on the tower, he will slide back and fall off the floor. For a while, he will continue falling, but he will teleport behind you.
5.Try not to block unless you have Perfect Block linked. He may break through your block if you get pushed back. Glides, Dodge Rolls (especially Block Dodge Rolls), and Haste are very useful for avoiding damage.
 Kingdom Hearts II Final Mix Data Rematch
Saïx's Data Rematch begins slightly differently from his first encounter. His Berserk bar will begin empty and he will immediately call to the moon and start charging it, to break this charge you must use a finishing move so it is recommended to un-equip any combo pluses prior to the fight; Negative Combos are not needed although it won't hurt you in this fight.
Once Saïx enters his Berserk state the battle becomes purely survival and waiting for him to drop a claymore, Saïx will take damage in his Berserk state but it is not without risk because of the large amount of damage he can dish out at any time. The best strategy is to keep as much distance between Saïx and Sora as possible and look for one of three things to happen: Saïx will stand in place and use his "All shall be lost to you!" super combo (which he uses very often in this fight); it is at this time that he is truly invincible and you must keep Sora away. Saïx may also begin his "Move aside!" combo, provided you put enough distance between Saïx and Sora you can simply stand in place and wait for his claymore to come to you. The third thing that will happen is Saïx will begin to run towards Sora, once close Saïx will either perform his "Begone!" combo or he will leap into the air and perform his super shock wave attack. Do not count on grabbing the super shock wave claymore, by the time you are close enough it will vanish and Sora will be close to Saïx and vulnerable. After every one of his combos, he will recharge his Berserk bar and the only way to stop this is to attack him. However, this comes as a risk because he will attack back with one of his combos except his "All shall be lost to you!" super combo because he needs a full Berserk bar to perform this.
After breaking Saïx's berserk state ensure to guard in order to block his attacks when you see them coming, they can still deal heavy damage and will allow Saïx to call to the moon to charge his Berserk gauge faster. Using the Magna Storm command does not reset Saïx's Berserk gauge in the Data rematch. The strategy should be repeated once you get the hang of it.