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Revised strategy for the laziest of people (such as myself): Unlock fake walls, block off all entrances but one, funnel all zombies into one hallway, extend that hallway around yourself so that it's closed off from the rest of the map completely and the only way to get to you is through the zombie spawn area (btw, make sure collision and demons are off). Then, stand at the end of the hallway, face the zombie spawn area, pull out your pistol, hold down the fire button, and click away from your browser to fire indefinitely. Zombies will funnel into the hallway and be killed by your infinite ammo pistol. Inevitably, some zombies will use this game's really bad physics to glitch through the walls, so you'll have to check periodically and kill them. You can essentially win the achievement automatically using this strategy.
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okay
set the mode to nightmare
to fast
no demons, collision
and use walls to block part of the way except for where you are shooting
easy win
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Turn on all of the things that the achievement says are allowed, block off one entrance of the first map with fake walls, block off the second one with a few layers (I did 3) of walls and a hallway in the middle for you to shoot through, take out the pistol, hold down space, then click away from the browser so the game auto-shoots with the infinite pistol ammo. You'll be able to go do something else until level 21, but because the zombies clip through the walls if they stack up enough you wont be able to leave forever. It shortens your painfully dull playing time for the achievement by a good margin, and if the game's physics weren't so poor the achievement would essentially be automatic after the initial setup.
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died because i was trying to act cool putting barrels around all the zombies than the devil shoots one of the barrels creating a chain reaction killing me
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Completely unplayable for me. When i start a round it says "paused". then about 30 seconds later it unpauses, my character is flashing, and he runs up and left endlessly until he dies