How do you defeat the last battle on kingdom hearts 2

The mst important thing is, make sure you keep on using Reversal when you can, and make sure you use Riku well when you free Sora, you need to block everything, every hit gets you further away from Xemnas so blocking for a little time wasted is better than getting hit and getting pushed futher even more. If you do these things, it should be okay to beat him.