Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Go Forth and Multiplay (10 points): Play an Xbox Live multiplayer match.
Nice Beating You (10 points): Play single player campaign mode, reach level 10 and defeat Blockbeard.
One Meeellion Blocks (10 points): Destroy a total of 1,000,000 blocks. All blocks you destroyed from all levels are counted.
I'll Show You Fits Of Rage (15 points): In any level, online or offline, kill 20 enemies within one minute.
You're Like Some Kind of Robot (15 points): Kill a total of 50 enemies with a longshot. All longshots through the entire game are counted.
Been There, Shot That (15 points): Use all 20 power-ups in the game, including the red power-downs. The tutorial level doesn't count.
Two Thumb Master (20 points): Finish the single player campaign.
On A Gamer's Diet (20 points): Shoot 50 zombies, 50 robots, 50 pirates and 50 soldiers.
Natural Born Gamer (20 points): Shoot 5000 enemies in total.
The Things I've Seen (20 points): Shoot every type of enemy at least once.
You Can Handle the Riot (20 points): Reach level 50 in endurance or co-op mode.
One Shot, Thrice The Score (25 points): Kill 3 enemies with one shot.