I'm currently running the new Xerneas in a deck similar to rainbow. Its built with Xerneas being the attacker and a lot of defensive pokemon to stall, occasionally attack and add damage to Xerneas's Rainbow force attack.
I'm also working with a Xerneas deck from Breakthrough, but not with stalling-Pokemon. I'm trying to build mine with all basic, non-EX Pokemon to get maximum use out of Brigette, and still have deck space for necessary trainers and energy. Sadly, though, it's impossible to fit in every type of Pokemon, and it's really hard to choose what types to include, as every Pokemon needs to have some purpose outside of just making Xerneas stronger. To just give a rough idea, I'll put down my picks. Overall, though, which Pokemon are included depends entirely on what I expect to go up against, so pre-tournament preparation is itself a huge roll of the dice. If I pack Pinsir and not Yveltal, then face zero Seismitoad and four Ghost-heavy decks, well that's how it goes.
It's also worth mentioning that the energy needs for this kind of deck are attrocious, so I suggest Exp Share to keep basic energy in check, and with Smeargle swapping for the type you actually need it's not so bad. That said, however, I took this to a store-tournament that was expanded and it fell flat to Hypnotoxic Laser when my opponent got nearly ALL of his KO's in between turns by poison, making this very much a Standard-only kind of deck. Didn't help that in that game my one-of Dedenne was prized (it was Yveltal), but that's how it goes sometimes.
Pinsir (Breakthrough), GCC for 100x2 to Seismitoad
Regice (AOr) because DUH, EX's are a nightmare
Smeargle (Breakthrough) Swaps basic energy, since I already use DCE and refuse to also use Rainbow
Dedenne (FuF) Doesn't need type energy, slaughters Yveltal/Rayquaza/Lugia, summons more Pokemon
Cresselia (Breakthrough) Free retreat with stadium, OHKO's non-megas on weakness with Muscle Band
Still searching...
Jirachi (Promo) would replace Registeel as type pick, Doesn't need type energy, says FU to special energy reliant decks
Also worth mentioning...
Yveltal (Breakthrough) shuts off tools (like spirit links) and can 2HKO a Shaymin as it sits on the bench
Registeel (AOr) disrupts EX