First, you have to obtain Korrina's Lucario, then have Mega Lucario face off against the champ's Mega Gardevoir.
Once you've proceeded to beat the game, you'll have fin stretch across the screen. Let the credits roll. In the post-game, grind on a rail, and right after that, go talk to Looker. When he asks if you're the champ of the Kalos region, say no. Afterwards, fish next to a rock.
You're welcome.
