The Elite 4's Pokemon range from levels 63 ~ 68, and there are no wild Pokemon over level 60.
The best place to train Pokemon is probably Sushi High Roller in Lumiose City, their trainer's Pokemon are all level 65. Use Experience Power and choose Triples and your Pokemon will level up in no time. It's going to enlighten your wallet though, one course costing you 500, 000 pokedollars. If you are looking to train lower level Pokemon, then Restaurant Le Wow is probably a better place, their Pokemon are all level 63, one trainer has an Audino too, which will produce a lot of Experience Points. The cost for entry is 100, 000 pokedollars.