Here's why and what you can do:
* Darkrai is the final boss: Darkrai is the final boss of the game, and defeating it is the primary goal. There's no "post-game" content where you can fight it again.
* It's not a traditional battle: The final battle with Darkrai is a unique experience. You don't actually fight it with your Pokémon. Instead, you use your partner's special abilities and other techniques to overcome Darkrai's power and save the world.
How to defeat Darkrai (in the story):
1. Follow the story: You'll encounter Darkrai after completing the main story and collecting all the "Wisps of Darkness" throughout the game.
2. Use your partner's abilities: The battle is more about using your partner's skills and finding the right strategy to overcome Darkrai's influence. This might involve solving puzzles, finding specific items, or even choosing the right dialogue options.
3. Be patient: The fight can be tricky, so don't get discouraged. The game does a great job of guiding you through the process.
Remember: The fight against Darkrai is a cinematic event more than a traditional battle, and it focuses on the emotional and narrative aspects of the story.