Vanitas is not too bad until he hops on the flying keyblades. I'm not sure if I guarded at the wrong moment, but I believe the ice is unblockable. My strategy was to always hit him before he can use them with a massive attack barrage using Ultimate Form.
You can't block the ice but you can dodge through it, and I found the most effective way to do that was to actually jump and airslide. If I was on the ground sometimes I could dodge one or two but the third (or fourth-fifth on the last volley) would still hit me. In the air they have a tendency to not track you but instead just curve down towards the floor so if you jump up and airslide you pass through the ones in the air then the others just crash into the ground. The trick is to make sure you can still get away from his charge attack immediately after so try not to double-jump or you won't fall down to the ground in time. Also his best opening, I found, is blocking his downward ice strike because that gives you a free counter prompt. It's also best to guard his large orb fire balls and get rid of them because otherwise they are a huge pain to worry about flying around the arena looking for you.
Marluxia- My favorite fight. The strategy I used to make Doom less of an issue was to stock a transformed Keyblade and break out the limit the minute he cast it on me and I was close enough to hit him. Generally that was good enough to get his armor down quite a bit and give me a bit of a buffer for the counter. So, I started the fight with my "reserve" Keyblade and once I got it to transform I would switch over to my main and continue until Doom.
Also, when he does his rose petal move with the four large circles I found actually gliding into the wall was pretty effective. I did that then wait for him to do his three whirlwind things, adjusting my flight path as the wind changed. Then, IMMEDIATELY after he finishes the third windstorm, guard and keep guarding until you get the counter prompt and wreck him.
The final 99 Doom count he does at 1 hp, the trick is basically just to not panic. He starts of doing a ton of his spinning attacks across the floor back and forth. Try not to block these if you can help it and rely mostly on dodging because blocking will also tick your timer down. Don't go on the offensive here, it's just a bait. Wait it out, don't panic. After he turns the floor black he will be completely open right in the center for you to get your last hit in. So the trick I think is to not panic, even if you get hit around a lot in the 'wheel' phase, you WILL have an opening when the floor changes, so make sure to immediately take it.
Also, just to note: any fight where they have a chance to reduce your max hp you can restore it back to full with an item or using a transformation (the few times I have thanked the lord for rage form).
Young Xehanort- I actually probably can't help much on this fight because I just sorta...killed him in one try so I didn't do the fight that much, oops. But I tend to be very block-heavy and I found blocking his whip move was really effective and often would give me a counter. The only time I dodged was when he made the clock on the floor because if you get touched by that you get hit pretty bad. See a clock face? Just dodge pretty much right away.
Terranort: You actually can guard the guardian. It also is a really really great opening to attack after he summons it. If you guard the second attack you can reprise through the third attack and get some good combos in. Also, his DM is basically just dodge-roll city. Run, watch the lights, dodge as you need to. Listen to the vocal cues to know when to dodge the lights his guardian throws directly at you in a line. I found with this fight being HYPER aggressive really helped. Also, guard his dark firaga and then immediately get ready to attack. If you can, use airstep to leap over to him (or just book it) right as you block the last stream and start laying into him. His own attacks stagger him a bit and if you are close enough to start attacking him before he recovers you can get a combo or so in.
Luxord: A lot of fun, love this fight. Sometimes you can tell which ones are the "real" cards because they turn a little...differently when you run past them. It's hard to describe, but if you circle around the arena and keep your camera focused on them you can kind of spot which ones turn a bit slower. Also worth noting that he actually has an opening (very short one) right after he does the attack where he shoots up into the air on a card...fountain-like thing? If you can block the first wave of cards and airslide through the next set you should be able to catch him in the air before he puts his cards up again to be invincible. Just nice to get an extra window instead of playing another minigame.
Master Xehanort: I found dodge rolling/airsliding through the spiky...ball things? tended to be the best way to get an opening. He usually opens up three or so portals and shoots out a few then I think on the fourth portal he shoots two at once, but after the second one hangs around for a few seconds. If you manage to slide through the second one and close the gap you can get a good combo in on him. Otherwise if you see him appear and throw a little dagger at you, try to block it. If you do, he visibly (and vocally) staggers and gives you a really big opening. Unfortunately if you are too far away to block it the attack still freezes you. I have only played it in Japanese so far so I don't know if the vocal cue is the same, but in my game he shouts "keyblade!" right before chucking it, if that helps you look out for that attack. His DM is basically a test of alternating between dodging and blocking.
Xigbar: Block to win. And do the shootout, it's great. His DM phase make sure to JUMP during the first attack. He pulls his guns out and spins in a 360 circle. You can jump over this. The next phase where he shoots from the ceiling I kind of just...ran in a circle around the platform and was fine.
Xemnas: Don't mess up blocking those lasers. Just don't. Keep in mind that the second and third set if you block the first volley there will be a slight break and if you guard during that break you won't be able to successfully block the second volley in time. I found to get around that issue was to dodge roll after the first volley, then immediately block the next set. Dodging is valuable because if he hits you with his vine things he can knock you into the walls he builds. One solution I had to that was dodging through the vines with a slide or roll. A plus to this is sometimes he was open on the other side for a combo. The other thing I would do is when he was taking the time to bother making all these walls around the arena I would run around smashing them just as fast so he couldn't use them against me.
I know I missed some fights but for the others I'm not sure if I'd have any particularly poignant advice that I can remember atm.