Easiest method to achieve the badge: Set nightmare difficulty, demons and collision off, boxy map. Kill the first wave of zombies in any way you want and leave one single zombie alive. Then choose a corner and put fake walls like this: http://i.imgur.com/m8UtoGA.jpg I did it on the two top corners for example, you can choose any corner. Then put yourself exactly in the corner and face the exit like in the image, grab your pistol and start shooting, switch to another window so it keeps shooting without pressing spacebar, and enjoy your free achievement without a single effort. My guide is better than others because pistol ammo is infinite and you don't have to recharge like other rocket methods, if you like my method please press the + to keep this comment up :D
Don't say my method is stupid without a good reason.
I admit my method explanation is not very clear as i wanted it to be, i'll write another comment with a better explanation.
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