Ok, so I have beaten the elite 4 and all of that good stuff.
I went to the battle Maison (both online and off) and nobody was there.
So, how do I battle there?
First grab the Pokemon you wanna battle with. Then go to the battle maison. Climb all the way up the stairs and go through the door. There are 2 ladies. Talk to the lady standing next to the PC. She will ask you to take the Battle Maison challenge. You will figure it out from there :P. Hope I helped!
U enter the battle maison and there's another entrance to the arena in between the Battle points trade mart. There's a girl standing next to the PC and you can talk to her to initiate your battle maison streak.
Hope I helped