I'd like people to take note that this comment is completely false and absolutely NOT how you play Geomancy Xerneas. You're handicapping yourself if you keep Xerneas locked up in it's Pokeball and try to 5v6, then send out Xerneas when the game is pretty much over.
What you're actually supposed to do early game is send out Xerneas liberally every time the opponent uses a dragon move or brings out a Pokemon that's set up bait for Xerneas. Then instead of mindlessly clicking Geomancy on turn 1 like a lab monkey, you do one of the following:
1) Use Moonblast/Thunderbolt/Focus Blast off the bat to soften up whatever check is coming in. Since every Xerneas check bar Ho-oh is either a phaser or a Steel-type, they're pretty easy to switch into afterward with another Pokemon while Xerneas escapes unscathed. Now you have a weakened check that likely wont have a chance to heal because they risk getting taken out by whatever you sent in, Xerneas will be able to break through later on, usually after 2/3 cycles, and just plow through the rest of the team, which often can't do a thing about Xerneas. As for Ho-oh, if neither SR nor Rain is up, it's a pain in the ass, but that goes for pretty much every other Pokemon in the game, not just Xerneas. Seeing Ho-oh on the other team doesn't translate to "lock up Xerneas and prevent it from contributing at all".
2) Double switch-in a Pokemon that has a good matchup against the opposing check. When Xerneas comes out, 99% of people will send in their Xerneas check, as it's just too risky not to, making prediction very very easy. It's often very obvious what the opponent's Xerneas check is, considering how they all have pretty similar traits (again ignoring Ho-oh). Now you have the opponent in a corner, they can either stay in and lose their check or come out and let whatever you send in get a free Draco Meteor/Water Spout/Earthquake or whatever.
3) Bluff a Choice Scarf set. It's important that if you're going to go this route is to be stubborn about it and not go out of character. Even if you know 100% that Aegislash is coming in, use Moonblast anyway. Eventually, your opponent will drop their guard and use their Xerneas check as fodder. If your opponent decides to call your bluff and keep in their Mega Mewtwo X or Rayquaza, then they simply die. Even though your opponent now knows you're a Geomancy set and your Xerneas is no longer in mint condition, you just sacrificed 50% of Xerneas to bring down another deadly sweeper like MMX or DDRay, I'd say you take that trade every day of the week.