You have to earn a total of 75,000 points in one Nazi Zombies game. I would play alone. Remember that it is the amount of points earned in a single game not the number of points you actually have at a given time. That means that you can buy anything you need without worrying about your points. When you get the achievement it will tell you immediately. Get Juggernog and a good gun. Don't forget to repair windows because you get points for doing so. Also, try your best to get headshots and knife kills because these give you the most points.