Honestly, there isn't really a good place to grind if you're having troubles with the Elite 4. The best thing to do would be to focus on your Festival Plaza and get the restaurants that raise levels.
Other than that, just focus on going through Poni Island. The levels of the trainers there are in the 50s, they don't have full teams, and the wild Pokemon are the highest level you'll find. For repeatable trainers, there really are none who are easier than the Elite 4.
