The type-exclusive Trainers who hang around the Battle Resort are good Exp. grinders, but you can only battle them once a day. There are specialists in Fairy, Grass, Bug, Ground, Water, Rock, and Poison, I believe. Correct me if I'm wrong. All of their Pokémon are level 60-ish.
A second good way the Exp. grind is to find the Mirage Spot with Audino. Defeating a single Audino gives upwards of 3,000 Exp., more with the Lucky Egg.