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

Serious Beginner (5 points): Complete any level in single player.
Serious Sam (20 points): Complete the single player campaign.
Are You Serious!? (30 points): Complete the game in single player on unmodified serious difficulty.
Kung-fu Fighter (10 points): Perform all possible finishing moves in the game.
Wanted Dead or Alive (10 points): Rescue professor Stein. So to say.
Get the hell off my ride! (10 points): Secure the bird.
Problem solver (10 points): Solve the riddle of the Sphynx.
Mission completed (10 points): Power up the Timelock.
The doorman should wear a suit (10 points): Awake the Guardian of Time.
Look, it's a secret (15 points): Find at least 50 secrets in single player.
Co-op Master (10 points): Complete the campaign in cooperative with at least 2 players.
Killer Jewelry (5 points): Kill an enemy using the Mutilator.
Headsman (10 points): Decapitate a Khnum.
Chain Explosion (10 points): Kill at least 5 headless kamikazes in one explosion.
Bone Crusher (5 points): Smash 20 Kleers with the Sledgehammer.
Maintenance time (5 points): Blow a Major Biomechanoid into pieces.
Trick Shot (5 points): Kill the kicked enemy while it is still in the air.
Load of Scrap (5 points): Rip Scrapjack's head off.
Wall of Bullets (5 points): Kill 20 enemies with the Minigun without releasing the trigger.
Classic Outfit (10 points): Find Sam's classic outfit.