The Regirock idea is my personal favorite, it also can combo worth with Ninja Boy (Go into a Xerneas, evolve into BREAK, the crowd goes wild). A couple things to add onto Regirock:
You can use Energy Switch to power up Xerneas BREAKs from your energy reserve
You can use Energy Keeper Carbink to help keep energy on Xerneas
Other things are Diancie-EX, Sylveon-EX, and possibly Xerneas-EX as partners, I only like Diancie and Sylveon (Kinda)
As far as an idea of a decklist here ya go:
Pokemon: 12
4 Xerneas
3 Xerneas BREAK
3 Regirock
2 Shaymin-EX
Energy: 14
10 Fairy Energy
4 DCE
Trainers: 34
4 Sycamore
2 N
2 Lysandre
2 Tech Supporter (Deliquent, Hex Maniac, Olympia, Brock's Grit, Pokemon Center Lady and an extra N all work)
2 Parallel City
1 Fairy Garden
4 Max Elixir
4 Trainers' Mail
4 Ultra Ball
4 VS Seekers
2 EXP Share
3 Energy Switch
Most of the card choices are pretty standard, other options are Float Stones, Super Rod, EXP Shares, and more Fairy Gardens