On route 14, just north of Black City/White Forest,there is a small length of road 3 spaces wide and about 15 spaces long, with girl standing on it. Get on your bike and go back and forth on that section of road for about ten seconds, until you see shaking grass. Get off your bike and WALK into the shaking grass. A wild Lv. 46-50 Audino will most likely appear. If you need to switch out to a stronger Pokemon, that's fine, but they're easy enough to take out. This should give you about 5000 or more every time.
Another easy way to get experience is in Nimbasa City. Every day, you can walk onto the courts and stadium and battle people, with Pokemon at around level 65. There should be about ten people there and you can repeat this everyday.
** Added bonus: If you put Pokemon in the daycare before you go looking for wild Audinos, you will get lots of steps to help level up the daycare Pokemon faster.