Try Victory Road itself
Defeating Pokemon 4-5 levels higher than you can get you quite a bit of exp. To start off you should turn on the Exp. Share. (If you want to) Next start by capturing Pokemon you don't have, this will help you fill up your Dex and get more exp. If you have already done that, or don't want to capture those Pokemon, start battling. Use any Pokemon you want to battle with. Make sure that you have plenty of status curing items, potions, and similar stuff. Once you have gotten your Pokemon to a decent level switch to the next Pokemon and start training it. When you have gotten all your Pokemon to levels you think are good, start going through Victory Road. If you train for a while then go through Victory Road you will be able to get more exp. than just going straight through Victory Road without training. If you think Victory Road is too strong, go a couple of routes back and train there.
Hope I helped!