to extend on this answer: Luxio, along with many other pokemon are exclusive to this method, and you can't just run around in the grass to find these pokemon. Instead, you need wait for a pokemon to appear in the grass. This comes in the form of tails, glowing spots, dark lumps, and other shapes swaying in the grass. Sneak up on this spot using the left direction stick (not the D-pad) to start a battle. And as Shiny Hunter Luna stated, you can tell what the pokemon will be before you battle it by its cry