Get a feel for a boss first. Go into a fight expecting to lose and just try to learn something about them when you fight. Just try to understand to boss’s telegraphing (how they communicate their attacks) through body language/sound effects and get used to the order in which they use their attacks. Don’t try doing this with lots of bosses at once, just focus on one or two at the time—the ones which you feel like you have a good chance at beating. Whilst their attacks can be intense, most of the battles are very repetitive and cyclical. Once you get used to the order everything happens in, it just becomes a case of perfecting your timing.
Maybe my controls were just jammy, but I found taking off Glide helpful for activating Aerial Recovery better. Again, I don’t know if it was just like a placebo effect/confirmation bias thing, but Aerial Recovery seemed to not respond very well until after I removed Glide.
I found the bosses much easier by taking off all my abilities which increased my combo, both ground and air. I guess it’s because you get to the finishers earlier and their damage adds up way better. Also, unequipp Combo Master (the ability which means your combo will continue even if you don't hit anything).
Form changes take time and you’re invincible during them. Rage Form is especially good as (as far as I can tell) if you rapidly press the Triangle function (forget the name; it lowers HP to increase attack or something; Rage Charge maybe?) and then use the finisher to exit the form, you can maintain invincibility from the moment of Form Change to the moment Sora reverts back to his regular form. This allows time for MP to recharge if you need it. Don’t worry if you don’t deal any damage to the boss—it’s just to cheese some extra seconds for MP recovery for a Curaga, some breathing room, to block an attack or what have you.
Take breaks! I managed to defeat 9 of the bosses in one day (way more than I expected), but when I saw the last two were ones I really dreaded fighting, I just put my hands up and said, “No more today!” In between times, I always sometimes cooled down by just messing around in the Data Greeting mode. Just don’t drive yourself to insanity trying to beat these guys because that just makes the whole thing painfully frustrating rather than… sorta-fun-frustrating.
Young Xehanort – He has an attack where he summons icicles twice in a row. Block the first set, but then wait. After he moves farther away, he summons the second. He then teleports closer to you—he’ll reappear either three times and use a Keyblade combo or four times and use his whip-attack thing. In either case, block when he reappears the third time. In my experience I blocked both the icicles and his attack (regardless of which one it was) 100% of the time by timing it this way.
Be cautious with him. He’ll sometimes combo then start spinning his Keyblade—hit him then and he’ll counters with the Stop attack. I beat him my third try so I don’t remember too much about him.
Once you ‘beat’ him, he actually retains a sliver of HP. If I remember right he uses his Stop attack here, and maybe something else?? Just be prepared to deal with him having his back to the wall since you might be expecting to finish him off only to have him survive. (At least I THINK he does this--he was the first one I beat so my memories have been all scrambled and faded fast since then).
Larxene – I felt like I beat her in about ten seconds just by going totally ham offensive with her; I’m sure it wasn’t that fast or easy but it sure felt like it. Unfortunately I’ve got no tips for her because of that. So my advice would be – go totally 100% offensive with her and don’t let up, I guess.
Saix – One of the battles where I found using magic was quite useful. The most vital thing I remember about his fight is when he goes full berserk and starts charging around, he’ll summon those spinning, flying claymores. Avoid them at all costs with a Dodge Roll because they can stun Sora, and a stun here can be deadly.
Vanitas – The main problem I had with him was his attack where he goes underground and bursts out, spilling fire everywhere. Afterwards he sometimes does a downward strike which freezes you. Took me ages to realize but: you can block all of these attacks. Right after his downward freeze-strike, he’s vulnerable, so combo him.
When he rides the Keyblades, I tended to try avoiding one barrage of ice and then using a Shotlock to protect myself from another whilst dealing a bit of damage too. That's just 'cause I sucked at avoiding them, though. You can still damage him when he's riding the Keyblades, so magic might be good too? I never used it, though.
I believe I used Oathkeeper against him.
Marluxia – I didn’t find him too difficult so honestly don’t remember much. I think I used Shotlocks whenever he went into an Armour phase. Also, like Young Xehanort, he’ll endure a final hit and put Sora under Doom again, only this time the counter starts at 99 and goes down SUER fast. He’ll attack viciously—I didn’t manage to block/avoid anything until I had like 12 count left and BARELY managed to get a final hit on him—I think with Firaga or Thundaga. Just be aware of his last minute bull honk.
Luxord – This is a battle of patience and waiting more than anything.
First, get the timing right with his card attacks. When he throws a card, it starts normal. When it glows, it speeds up, so block. Get used to blocking them at the right time because he doesn’t summon them one after another. Sometimes he summons three cards and, if you’re too close, two will bypass your block and land on the floor (they won’t hurt you though). The fewer cards on the ground, the easier a time you’ll have.
The red cards he summons here can be stopped with Thundaga initially. I liked to follow it up with a Curaga personally since by the time the attack comes around again, your MP should be back. I didn’t want to worry about suddenly being unable to use Thundaga when the red cards appeared, but this is possibly me being over-cautious. I believe after he uses his… are they still called desperation moves? Basically when he cuts the battle with the short scene, the red cards then become another element—Fire, I think. Push comes to shove, you can easily avoid these by Dodge Rolling out of the way, but they do end up on the ground this way.
When he summons the cards which spin around him, wait until you see the one with the circle, then run in the direction opposite the cards spin. I think the cards spin clockwise, so what I mean is, run counterclockwise. If I’m mistaken, just reverse the directions. If the cards are still moving very quickly, don’t be afraid to time your attack ‘early’—attack when the card BEFORE the circle card appears. When they’re moving REALLY fast, you’ll connect just as the circle card comes into range. Remember the process of elimination, too—if he has four cards out and you’ve seen three bomb cards, the last one approaching MUST be the circle. If there is only one card left, use Thundaga.
When he plays the Hide and Seek game where all the cards on the ground move around then stand up, move to one side of the arena. Once they’re stood upright, move to the other side. The circle cards WON’T adjust their position with the camera. You should be able to tell either by their face being visible to you or their odd angle compared to the rest of the cards which ones have circles. If you take too long, a card will fall over and glow, move beneath Sora then explode. Don’t worry too much about the explosion since they can be Dodge Rolled (and maybe jumped/glided over?), but if you take TOO long (I think after three cards have exploded, if there are enough) all of the cards explode.
When you play the game where you’re surrounded by cards which flip over and you have to find Luxord, DO NOT ATTACK HIM. Use the Triangle function instead. Otherwise you’ll probably (like I did the first few times) just hit an adjacent card and get Sora blown up. Be prepared to Dodge Roll at a moment’ notice since he’ll frequently attack with the cards in a tower. I don’t know if you could Block this instead, but I wouldn’t count on it.
Xigbar – Like Luxord, you gotta be patient since you’re playing on his terms. Guard is your best friend and absolutely 100% necessary for this fight.
A few things I recall: when he shoots a barrage of red bullets into the air, ignore them. He fires a must faster barrage afterwards, so block those instead.
If he shoots a single blue, bouncing bullet, ignore it. Wait for the next barrage of red bullets and the timing should be spot-on so that you Block both the red bullets and the blue at the same time. If he shoots three blue bouncing bullets, start Dodge Rolling because he’ll start firing blue projectiles instead of red ones and those can’t be blocked.
If you manage to deflect one of the blue, bouncing bullets, they do deal a pretty decent amount of damage to him, and I believe they stun him and leave him vulnerable to a combo? But I only managed to do that once, so I’m not sure and my memory sucks.
When you go into Show Down (?, the quick time event thing) with him, I found it fairly easy. If you struggle with it, I’d maybe suggest ignoring Xigbar entirely and just paying attention to where the ‘Press X’ retaliation command appears in the lower center of the screen. The moment the message appears, press X. It doesn’t really matter what Xig’s doing so long as you respond as quickly as possible.
When he summons the circle of arrows that widen around the battlefield, try to position yourself so that you’re blocking his red bullets as the wave washes over you and you’ll Block the arrows too. Alternatively, you can Dodge Roll through it.
When he uses his ‘Desperation Move’ and the arena goes really tiny, I basically just Dodge Rolled a bunch. It took me a few times to get the rhythm right and avoid getting hit but this attack is way less scary than it might look.
Dark Riku – I remember just going really hard on the offensive for this one. His attacks are immense and fast. Most of the time I Dodge Rolled, can’t remember what if anything can be blocked. Sorry. Don’t recall much here, but I think I used Oathkeeper.
Ansem – I remember him being relatively easy, so can’t recall much about the battle. The most difficult part is when he changes the field and summons a pair of large spheres which shoot off projectiles. The projectiles can inflict status ailments so make avoiding them your priority.
Xemnas – His fight actually felt very similar to his KH2 incarnation to me. Maybe it’s just been a while though? He’ll use the same long-ranged thorn attack which can’t be Blocked. And… I forget everything else, lol.
Terra-Xehanort - His Dark Firagas can be deflected with Block and the damage they deal isn’t half bad. What a dork.
His combo should be Dodged Rolled rather than Blocked, since when his Keyblade is red, you… can’t Block it. And his Keyblade will always be red at the start of each combo. He’ll combo four times—I don’t remember if this is occasionally or if it’s all the time? It might only be three times when the fight starts out and four when he gets more aggressive. Either way, after his combo chain, he’s vulnerable to an attack, so combo him.
When he changes the arena so it’s surrounded by light pillars and he sends the Guardian to attack, Dodge Roll seems to be the best bet. The first time I fought him, I somehow managed to Dodge Roll INTO the red light beams around the circumference and not take any damage from them or the Guardian, but thereafter have never managed replicate this result. Weird. Focus on avoiding the spiraling blue beams near the center as they’re constant. Move to the outer edge of the cicle as they close in, then back toward centre as they move outwards. When Guardian appears, just make sure you’re not avoiding one attack only to and roll into another. Try to roll past both at the same time, if you can.
Xion – this battle is FAST, FAST, FAST. She summons light beams which don’t deal damage, but they lower your max HP. If your Max HP is brought down to one, being hit by a light beam WILL kill you. Block them. You can Dodge Roll, but they come so fast that Blocking is better. The HP cut wears off after time and also gets reset if you die and use a Kupo Coin.
She has a few variations of the light-beam attack. Take some time to get used to each. Sometimes it’s like five in a row, other times she’ll fire one, teleport, vanish again, then fire another the NEXT time she appears.
When she goes into an Armour phase, I usually just forgot about attacking her and focused on avoiding her attacks. If I reacted fast enough, a Shotlock from Ultima could take her Armour down quite far and I’d usually be able to chip off the rest. The Oathkeeper Shotlock was also pretty good, too, and a couple of times I managed to get rid of her armour using it twice in a row. Again, she's fast, so this usually seemed way riskier than was worth it.
Honestly I found her fight to be the hardest out of the bunch. It’s really fast-paced and one slip-up can cost you the battle. I know that probably sounds applicable to a lot of these fights, but with Xion I really felt it. It took my me about 2.5 hours of constantly fighting her to finally win, so I guess… the best experience is just to die again and again and again. Surprisingly she's one of the few I didn't need a Kupo Coin for to finally beat, though.
Master Xehanort – you'll open on a standard field but it soon changes to a field of lght after the first few attacks.
After the battle opens he teleports into the sky and attacks with dark spheres and light lasers. Dodge Roll out of the way. Afterwards, he will reappear on the ground close by and say either “Keyblade!” or “Freeze!” (or something similar, I forget…). Get fairly close to him and block his freeze attack. If you do, activate your reprisal (make sure it’s one that draws Sora close to the enemy) and dish out combos on him until he vanishes (for me, with no Combo+ abilities equipped and Ultima Weapon, I could get a combo and a half off on him before he vanished). There’s a pretty decent gap between his previous attack and whenever he uses this Freeze, and it’s one of the optimal moments to attack him, so definitely focus on getting the timing and sound cues right here.
When he summons the spinning gear-ball things, doing Dodge Rolls toward him worked best for me, but sideways or if your timing is good, away from him can also work. The problem with these are the fact that they can freeze you, which can be utterly deadly if he gets you with one of the first ones. However if one DOES hit you and you aren’t frozen, if you’re in the right place at the right time, you might be able to recover and use your X button counter and get a combo in on him. Don’t rely on it, though.
In the standard field, he’ll use an attack where he shouts, “Give me power!” (I think) and the large cannons which shoot red beams appear. After the… second, I think, beam fires, he’ll actually appear in the centre of the area. If you remain close to the middle whilst Dodge Rolling out of the beams’ way, you can start a combo on him. You might get hurt by the beams, so it can be risky—but Ultima form was super useful here. If you can activate it, he’ll be staggered and get thrown into the air. You can then immediately go into a combo on him. Other Keyblades may have the same stagger effect when you trigger the transformation, but I didn't use anything against the Data 13 except Oathkeeper and Ultima.
After getting to about half HP, he'll change the arena again and light-lasers will appear and attack. He then sends an onslaught of shadow-copies, kinda like Vanitas, to attack pretty viciously. I think you can Block them but I don't tink I ever managed to do it and just abused the Form Change tricks. Once he's finished, the arena returns to its basic form again. I believe it's after this that he gains the ability to lower your max HP kinda like Xion does, so be cautious of that, too.