Grrr... don't use AR. It's a nasty invention that ruins the fun of the game.
After you beat the E4, nab something with Defog and go back into Victory Road and into the cave that originally the fat guy was blocking the entrance to. Use Defog because it's awkward in there if you don't. You'll team up with Marley, who is a very annoying and ungrateful little... anyway. You'll go through the cave with her and her Arcanine which uses nothing but Helping Hand, so get to the end and be rid of her as soon as you can. Once you exit the cave there'll be some high levelled Pokemon in the grass and at the end of the route there's a white rock. If you've got Oak's Letter from the Shaymin event, this is where it appears and you run after it. If not, well, just grab the Destiny Knot and run away.