Geomancy is incredible. I'd suggest forgoing your item and getting a Power Herb just for the sake of using Geomancy. As long as you're in a good position to sweep and it is unlikely you will be forced out, the boost which is basically the equivalent of two Spec and two Scarf boosts (with that Special Defense boost added on also) makes Xerneas a beast who can quite easily sweep the foe.
It's a bit less useful without the use of Power Herb. The problem really is that the turn you're inactive allows the opponent time to switch into an effective counter. It's still a solid move, as if you see that your opponent has no solid counter or if you are able to KO that counter earlier in the match, now you can just run away with the Xerneas sweep. And it's not like Sky Attack, where you just switch into a type resist to reduce effectiveness. So I'd just recommend it on all Xerneas (within reason. Useless on a choiced Xerneas of course.)