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

  Environmental Awareness (5 points): Kill a zombie using an environmental 
  hazard in any game mode. 
  Gas Powered (5 points): Complete an entire arena, getting every kill with the 
  Chainsaw. 
  Gun Shop (5 points): Kill at least one zombie with each weapon type in any 
  game mode. 
  More the Merrier (10 points): Kill 10 or more zombies at the same time in any 
  game mode. 
  Socialite (5 points): Play in an online multiplayer game. 
  Tour of the City (15 points): Complete each arena at least once in the same 
  game in any game mode. 
  Can't Touch This (20 points): Rack up a 100x multiplier in any game mode. 
  Friends in Low Places (20 points): Save 25 survivors in any game mode. 
  Interior Decorator (20 points): Kill 25 zombies in any game mode using 
  environmental hazards. 
  A Week To Live (25 points): Complete 7 days in any game mode without dying. 
  Hotter Than Hell (25 points): Complete the final day of the "7 Days of Hell" 
  unlockable mode. 
  Anniversary of the dead (45 points): Complete the Final Day, in either Single 
  or Multiplayer, and finish the game.