When you start the game you're already a level 40, so you only have to gain an extra ten levels for the achievement. It took me less than a playthrough and a half to reach 50. If you want it quickly, kill lots of the special characters like security guards and wranglers who give you extra PP. Also saving survivors gets you extra PP. Finally when you first enter the harvest room you'll see a pit with zombies in it. Next to that is a control panel and if you turn it on, it will burn the zombies below and also net you 5000 extra points.