EV training. I assume you're not gonna Super Train it for half an hour, you probably don't need a guide for that.
The fastest way to EV train is with Sweet Scent to summon horde encounters and a move that hits all the opponents, along with infecting your Pokemon with Pokerus and equipping a Power item (ex. Power Anklet increases EV gain by +4 speed per opposing Pokemon fainted.) These Power items can be gotten from the Battle Maison at 16BP each.
You can check the locations of the hordes and their EV's at the following thread in Smogon:
I use the following personal favorite sites to get Attack and Special Attack...
Route 14 - Laverre Nature Trail
Bellsprout, Ekans - Attack
Vanillite, Smoochum - Special Attack
Route 20 - Winding Woods
Trevenant - Attack