Ok i'm trying to get my Pokemon to all Lvl 55 before I try to take on the Elite 4 but I cant find a good place to train and get good exp.
Here are my Pokemon, they are all LV. 45 - 50.
Lucario, Genesect, Eelektross, Samurott, Braviary, Camerupt.
LOL, I'm already on Level 54-58 and I'm still at the 8th gym :P
But the best place is on Route 9. The Audinos there give loads of EXP. Equip a Lucky Egg and you'll level up very quickly.