Step 1: Buy a bunch of Super Repels/Max Repels.
Step 2: Go to the area in your game with the highest level of wild Pokémon.
Step 3: Find the smallest patch of tall grass, and use a Repel.
Step 4: Run back and forth in the grass until you see one of the tiles start shaking.
Step 5: Go to that patch of grass. Most of the time it will be an Audino.
Step 6: KO the Audino. Audino give a much larger amount of Exp than any other Pokémon.
Step 7: Rinse and repeat until your Pokémon are at the level you desire. If you have a Lucky Egg, equip it.
Grinding is boring, yes, but this is the best way to level up in the Gen 5 games. Alternatively, find routes with Breeders on them. Fight them, leave, and come back. They will challenge you every time you come through an area.
