Accomplish the indicated achievement to get Gamerscore points:

Co-Op: Successfully complete Wave 5 with another player on Xbox Live on the default settings.
Co-Op Master: Successfully complete Wave 10 with another player on Xbox Live on the default settings.
Dodge: Survive Wave 1 for 28 seconds without dying on the default settings.
Glue: Successfully complete Wave 1 without moving or dying on the default settings.
High Score: Get first place on the game score board on the default settings.
Save All Humans!: Save all humans on Waves 1 and 2 on the default settings.
Time Challenge: Reach Wave 4 in under 45 seconds on the default settings.
Versus: Get a rank of 5 in Xbox Live Ranked Versus mode.
Wave 5: Reach Wave 5 on the default settings.
Wave 10: Reach Wave 10 on the default settings.
Wave 30: Reach Wave 30 on the default settings.
Wave 100: Reach Wave 100 on the default settings.