Gah, I hated Axel's fight. Victory basically depends on getting the Reaction Command (think it's called Burst Frontier) to work.
I've fought Xemnas so often that I've worked out my own little strategy. You'll need Second Chance, Once More, MP Rage, and a well-developed Limit Form. He starts the Skyscraper stage with a long chain of lightsaber attacks, so Glide away unless you want to de quickly. Then, activate Limit Form and play it defensively by blocking and counterattacking, since LF's counter, Zantesuken, can slice off a ton of health at once.
If he manages to hit you, use Infinity (Ragnarok). The initial combo should restore a lot of your health, and if you manage to get Giga Impact on the follow-up, Xemnas will lose several bars of health as well as get stunned. Be sure to not let your MP run out, though. If you have MP Rage equipped, this shouldn't be a problem; you get healed by Limit attack, Xemnas hurts you, you gain MP, use Limit to restore health. It's a vicious cycle that actually helps you out. Run out of MP and have low health? Glide, Dash, or just run like a coward til the MP's recovered.
Now, Final Xemnas doesn't start the fight by flinging you into the air, but instead goes straight to his double electricity spheres. I've found that strafing him, then Dashing towards him (y'know, that ability you get from Wisdom Form?) when he creates the first sphere, then using Reflaga when you're right next to him usually gets some decent damage in. If he starts that aerial combo (the one that he starts the fight with in the main campaign) and your Reversal fails... do nothing. As long as you have Second Chance, Once More, and you don't hit the floor, Xemnas can smack you around like a tennis ball and you won't die. Have a Curaga ready for once he's done.
Now, later in the fight, Xemnas will start shooting laser beams at you like he got a laser blaster from Star Wars. I usually opt to go into Limit Form and Strike Raid he hell out of him. It doesn't do as much damage as Infinity, but with him flying about quickly and with all the laser beams, Strike Raid will probably be more likely to hit at all, thus restoring your HP.
Oh, and keep an ear out for audio cues. If you can pick up on sounds that usually precede something very bad, you'll have some time to react to it. For example, when you hear "can you spare a heart?" be ready to heal yourself, because Xemnas is about to trap you in a life-draining spell and you'll want to give yourself as much time as possible.
... Sigh, that life-draining spell. It's usually the thing that kills me now, because Riku's a pain to use. When you switch characters, Xemnas will send a clone of himself after Riku (you) to knock you away from Sora. Dark Firaga his every time he even gets close and keep running.
After that, the fight's mostly Xemnas shooting lasers and the odd electricity sphere. Oh, and the infamous laser sphere at the end. But otherwise, he's not so bad. Roxas, on the other hand...
