How to get the battle park in Pokemon Diamond?

You can't get a "Battle Park" in Pokemon Diamond. It's a feature introduced in later games, like Pokemon Black and White.

Here's what you can do in Pokemon Diamond to battle other trainers:

* Battle Frontier: The Battle Frontier is a post-game area with various battle challenges. You can battle trainers here to earn Battle Points and unlock new facilities.

* Battle Tower: You can challenge trainers here to earn Battle Points and unlock new rewards.

* Trade and Battle with Friends: You can trade and battle with friends who also have Pokemon Diamond.

If you're looking for a similar experience to the Battle Park:

* Pokemon Black and White: This game features the Battle Park where you can battle trainers and collect rare Pokemon.

* Pokemon Omega Ruby and Alpha Sapphire: These games have the Battle Resort, which is similar to the Battle Park.

I hope this helps!