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

Exterminator (10 points): Defeat the giant spider.
Sharp Shooter (10 points): Don't waste more than 30% of your bullets.
Unstoppable (10 points): Finish a zone without losing a life.
Lair Slayer (10 points): Complete all underground zones.
Annihilator (10 points): Kill more than 85% of the total enemies in a zone.
Small Arms Expert (20 points): Finish a zone without using grenades or nukes.
Twin Medal (20 points): Finish a zone in Co-op mode.
Speed Freak (20 points): Finish a zone in less than 15 minutes.
Peace Maker (20 points): Finish a zone killing less than 50% of the enemies.
Tricked Out (20 points): Max out all of the vehicle power-ups.
Survivor (20 points): Finish a zone killing 60% of the units on foot.
Assault Hero (30 points): Complete the game.