Go to the nearest grassy area on Victory Road, run around near the grass until a patch shakes, and walk in it. If a Pokemon called Audino appears, defeat it. The give a TON of experience points. Also make the Pokemon you are training hold the Lucky Egg that Professor Juniper gave you in Celestial Tower. Once you defeat it, repeat the process, then when that Pokemon is trained, give the egg to another Pokemon. Rinse and Repeat.
Buy some revives and Full Restores when you go to challenge the Elite Four. There is a building next to the entrance to fight them where you can buy items and heal Pokemon.