Pokemon Ranger: Shadows Of Almia
This game is titled Pokemon Ranger: Shadows Of Almia in North America, Europe, and Australia, and Pokemon Ranger: Batonnage in Japan.
Successfully complete the main storyline. You can now ride Wailord from Haruba Village to reach the Capture Arena.
Fight Darkri again
After completing the game, go back to the top of Alturi tower. At the top, there is a man. Speak to him and you can fight Darkri again. However, once you have fought him you will lose all your Pokemon.
Strategy guides from GameFAQs