Simply go to the multiplayer menu, then to the leaderboards, then choose any class. Inside only the first 100 races will let you DownLoad a ghost, by choosing any race and pressing the X button. You don't even have to race against it. just D/L it and accept the challenge to play now and the achievement will unlock before it stops loading the race. You can combine this with other achievements as well. I personally got "Can't Stop" while racing this.