I'll explain my way in steps:
Step 1: Put two of your Pokemon in the daycare. Make sure they won't breed.
Step 2: Go to Centrico Plaza in Lumoise City.
Step 3: Get on your bike.
Step 4: Tape your Circle Pad down.
If done correctly, you'll start going in circles forever. Leave your 3DS for a while, and when you return, your Pokemon in the Daycare will be higher level.
IMPORTANT: Make sure you don't have an egg with you, as it hatching will cause you to have to take off the tape and put it back on.