omfg this was the hardest fight ever. Whenever you take control of riku, press square constantly to dark aura the clone back further, until you can make it up to sora in time to save him. Use reaction commands, and if you are low on health, FLY AWAY from xemnas, and wait for riku to heal you, unless you can use curaga. Unleash combos, and use the dark shield command and reverse a lot. During the last portion of the fight, xemnas will shoot lasers, jump and move to avoid them, although chances are you will still get hurt. During the last bar of health, a battle cutscene will appear, and a huge laser horde will come after you, you and riku need to press triangle and X as fast as you can to deflect them back, if you succeed, you knock xemnas unconcious, with 1 tiny spec of health left. to defeat him, land a combo and you're done ^_^