lol Sorry got confused, thought you meant kh2 xemnas.
Anyway what level and difficulty are you on? Level could be a factor.
As for strategy I mostly just guard and use the counter against Xemnas since all his attacks can be blocked by guard. As for his move where your trapped in the box just have high jump equipped or flowmotion out of it.
So my strategy for him is largely dodge, guard & counter.
As the DE's build up their link gauges I'll use link attacks as well, even if they hardly hit they still give you a invincibility window.
Another strategy is the Balloonga spell, one or two of those are great for this fight (or any fight really).
Oh and there's a brief moment to reality shift him if your quick enough. If done right it'll stun him long enough to wale some attacks on him as well.