Accomplish the indicated achievement to get Gamerscore points:

Anti-Thief: Kill a thief.
Bling: Collect 10 treasure chests on Xbox Live on the default settings.
Co-Op: Successfully complete level 10 on Xbox Live with another player on the default settings.
Co-Op Master: Successfully complete level 20 on Xbox Live with another player on the default settings.
Elf Master: Reach first place on the game score board with the Elf on the default settings.
Game Master: Successfully complete level 100 on the default settings.
High Score: Score 100,000 points in an Xbox Live Co-Op game on the default settings.
Max Co-Op: Start a four player Xbox Live Co-Op game.
Treasure: Get 11 treasures and complete a treasure level.
Valkyrie Master: Reach first place on the game score board with the Valkyrie on the default settings.
Warrior Master: Reach first place on the game score board with the Warrior on the default settings.
Wizard Master: Reach first place on the game score board with the Wizard on the default settings.