Pokémon Diamond and Pearl/Route 203

Event Battle: Your Rival
Your rival is waiting for you near the entrance from Jubilife City, waiting to show off his battle skills with his new Pokémon. He has picked the starter that trumps your starter's type, so use your starter to take down his first Pokémon, and rely on the new additions to the team to defeat his starter.

Moving on to Oreburgh City
After your rival is defeated, he will run east to challenge himself at the Oreburgh Gym. To get to Oreburgh, you will have to fight several Trainers and pass through a short cave passage. Don't try to do this all at once; fight some Trainers, catch a few wild Pokémon, and return to Jubilife for healing before going to Oreburgh.