Instructions
Route 210
Travel to Route 210 and journey to the northern half. Locate two long stilts that form a narrow bridge.
Look for two long stilts and use a bike to cross the bridge. Be aware that you will have to defeat a ninja at this point in order to continue on your journey.
Find the second, similar bridge, which should be visible after successfully fighting the ninja, and ride a bike across it.
Locate the rocks to your left as you exit the bridge. Use Rock Smash to crush them open.
Collect the TM30 Shadow Ball that was hidden behind the rocks by moving towards it.
Exchange at Fight Area
Locate a sailor in Snowpoint City. Upon speaking with him, he will ask if you would like to go to Battle Park. Accept his offer by selecting "Yes" and he will automatically transfer you to Battle Park.
Enter the Fight Area and move to the center of the Frontier.
Enter the Right Exchange Counter, which is accessible through the Left Exchange Counter entrance as well.
Follow the instructions at the shop to purchase Shadow Ball for 64 Battle Points.