The Elite 4 in Kalos requires almost no preparation. Just do the cool thing that wins practically every battle in-game: use super effective attacks! You have one of the greatest Pokemon in the game on your team, and it doesn't even need Geomancy to be good. As a matter of fact, it'll just waste time. Don't use it.
Everything except Dragalge will die to Xerneas' Moonblast in one hit. Dragalge will more than likely take two, plus it can hit Xerneas super effectively, so you may want to try any one of your Pokemon with a Dragon move.
You see that Tyrantrum you've got there? And you see how all of her Pokemon are weak to Rock? And how she only has one move that can hit you super effectively? Do I really need to explain this?
Murder everything that Wikstrom knows and loves, except Probopass, with Mega Charizard X. Murder Probopass with Lucario's Bone Rush, which bypasses Sturdy and is 4x effective.
Gordon Ram- I mean Siebold
It might just be me, but his Clawitzer is incredibly annoying. Don't bother messing around with it, kill it with Xerneas' Thunderbolt. Speaking of Thunderbolt, you can do the very same thing with all the rest of his team. Fascinating how the best Trainers in the region have an easily exploitable weakness.
Three, yes THREE of the Kalos Champion's Pokemon, the region where Fairy was introduced, are weak to Fairy. Xerneas eats the souls of Diantha's Tyrantrum, Hawlcuha, and Goodra with zero regret, and it can sweep the rest of her team as well if you so desire. But for the sake of type matchups, let's say you want to play it safe (lol). Aurorus goes extinct again, courtesy of Lucario's Aura Sphere. Gourgeist is ignited like the pathetic little Jack-O-Lantern it is by Charizard's Flamethrower. Steelix eats Mega Gardevoir for breakfast (or lunch, dinner, dessert, or even the fabled brunch, depending on the time of day).
As you can see, the Kalos League is extremely easy. Have a basic idea of what to expect going into it, and come out after murdering the beloved of the Elite 4 and Diantha.
Hope I helped!