So far, this must be a battle between XY and ORAS. when you battle, you play with Kalos Rules. These rules make any new moves unusable, any new items unusable, and any of the new megas unusable. Pokemon like Primal Groudon, Primal Kyogre, and Mega Rayquaza are all unusable.
Annoying right?:(
Source- too much experience from friendly battles