Xemnas Boss Strategy
HP: 1500
Location: Altar of Naught and beyond
Boss Difficulty: ***
Recommended Level: 50+
Suggested Forms: Valor, Master, Anti
Drive Form Levels: 2 or 3 for all of them except Final Form maybe
Recommended Keyblades: Sleeping Lion (Default), Oathkeeper (Master), Oblivion and Guardian Soul

Xemnas Form 1
Okay, Xemnas will transport Sora outside Memories Skyscraper, this fight is pretty easy, Xemnas has several health bars however don't let this put you off. Despite his health he's actually quite weak in this form. My recommended Drive Form is probably Valor Form at about Level 3, and Sora should be around level 50, Valor Form's Drive Guage depletes much slow than Master Form, meaning you can unleash an arsenal of attacks for a longer period of time.
In Xemnas' first form he will sprout two beam swords from the palm of his hands which he majorly uses for attacking, these are hard to dodge but due to the fact they do extremely little damage that doesn't really matter. His next attack is Reflect, when he uses this you can't run into him otherwise you'll recieve damage so just wait until the wall begins to fade then begin unleashing combos on him once again. His final attack is when you get surrounded and Xemnas disappears, I found it pretty tricky to find out what to do but you have to run to the top of the steps leading to the entrance of Memories Skyscraper and use the Triangle Button to do a Reaction Command, Sora will lead up the side of the building and by pressing Triangle again challenge Xemnas in mid air (like with Roxas and Riku on Deep Dive).
Just keep unleashing combos and you should have him sorted quite quickly.
You will now be given a chance to save the game, save the game and enter the door through the door you will find Dark City. Just keep jamming Triangle Button and do the Reaction Commands until you reach Xemnas' Second Form.
Xemnas Form 2
Xemnas' second form is completely different from his first, he will be wearing some armour and basically just sit on a throne unleashing the odd attack on you. Again he has lots of health but if you just keep smacking him with combos it shouldn't really be too difficult because Riku will heal you when your health begins to get low.
Upon defeating Xemnas whilst he is on the throne Sora and Riku will be catapulted into the air, you now have to take on a flying task on a jet.
The basic idea is that you shoot the parts of the Castle That Never Was and destroy it, if you see bullets flying towards you use Absorb, that way you can Charge your Megalaser however more powerful attacks like Lasers can't be absorbed. Just use X to keep on attacking and when your Megalaser is fully charged use that to blast away at the Castle That Never Was. The Megalaser will fire a thick beam from the left hand side of the screen to the right, so it is more preferable if the CTNW is too the left side of the screen or on higher health when you use your Megalaser.
For the third and final part of his second form Xemnas takes on the form of a knight in his throne again. The same goes except this time he will use the same Reflect move that Xemnas' first form has, to dodge this attack don't go near him when the Reflect barrier is strong, but when it fades use the Square button to glide towards him then unleash a combo on him. Note that the only way to get near enough to him in time before he starts attacking him is to Lock on to him and Glide towards him, going into Master or Valor Form will make this process quicker.
The difference this time is that he will catapult you out into space and send buildings hurling towards you, the best way to get back to him is the use the Square Button to glide towards one of the buildings that is suspended in the air and
not moving and press Triangle several times, this will lead to a series of Reaction Commands which will allow you to take a hefty strike on the CTNW.
Note: You may wind up getting Anti Form during this fight, however it is extremely useful whilst fighting Xemnas' knight form due to the fact you can rapidly keep hitting him with combos with been knocked to far back, and you can recover quickly using Ariel Recovery then return to him fast due to Anti Form's immence speed!
In doing this you should eventually be able to beat Xemnas' second form, now enjoy a short cutscene with Xemnas talking to Sora and Riku and get ready for the final boss.

Xemnas Final Form
Surprisingly Xemnas' Final Form isn't so hard. It's very similar to his first form but with many more moves and it's much more powerful.
As soon as the fight begins a scene will be initiated with Xemnas using an ariel attack on Sora, just keep bashing Triangle and X buttons to keep attacking Xemnas and use a Reaction Command everytime the oppertunity comes along. Once you are out of this attack you can actually get attacking Xemnas' Final Form.
His most noteable new ability is that he can clone himself and move around, I suggest you take out the fake Xemnas and use Master Form straight away. Whilst in Master Form you can unleash some hard hitting combos on Xemnas but also you can use uber powerful Magic Spells. I suggest hailing him with
Thundaga or
Blizzaga spells, because these will do some extremely high damage. But I wouldn't use all your MP because you need to some to use the Curaga ability later on in the battle.
If Magic fails unleashing a huge combo, Master Form style will always work!
His new attacks are one in which thousands of Beam Swords fire towards Sora and Riku, the easiest way to survive this attack is to keep jamming X and Triangle at the same time. X button reflects the beams with Sora's Keyblade whilst Triangle reflects the beams with Riku's Keyblade.
His last attack is when he captures Sora in an energy ball and begins lowering his health, then you gain control of Riku. It's difficult to get around to Xemnas because several clones using Reflect will keep knocking you back but if you run sideways around the hole in the center and around the Xemnas Clones you will reach the real Xemnas. When you get to him press Triangle to free Sora then continue pressing Triangle to unleash an insanley powerful joint combo between Sora and Riku (Reaction Command).
And with that said you should be able to easily take on Xemnas' three forms, good luck!