Go the the news reporters, as Stupid Nino said. Every time you battle them, just say no when they ask for an interview. They are located firstly to the East of Mauville, North of Mauville, and East of Fortree. Give the Pokemon you want to train Exp.Share, and pair it with a strong Pokemon. It will gain just over 2000 Exp points a time, so just keep doing it over and over again for an effective training method.
Thanks to Galade for the signature.