Note: This game is also titled Pokemon Ranger: Batonnage.
Successfully complete the main storyline. You can now ride Wailord from Haruba Village to reach the Capture Arena.
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.