Fighting, a lot of his team has stone edge though so it's difficult to fight with flying types which was a pain for me as I used Swoobat so his two main weaknesses were wrapped up in an easy to kill package
Audinos are a good way to train if you haven't already been using them. Just find a patch of grass with the highest level pokemon you can find and run along the path (but not in the grass) until a patch starts shaking and then kill the Audino and you'll get a fair bit of EXP with a lucky Egg equipped.
Oh, Krookadile's destroy the elite four if you have one.