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Use the first several levels of the boxy level to get a combo of around 60, then just use fake walls to make a one unit wide path across the top with the left side open to below. Then stand on the right side of it, using the pistol and its infinite ammo. Position your arm down the middle of the corridor then get a book or something and lay it on your space bar then leave the room. The zombies are polite. They will not run over zombies in front of them. Those coming from the top will wait for an opening below them before entering the area. So they'll take their turn, with those behind waiting patiently to be mowed down by your tiny pistol.
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Zombies won't attack barrels. On the Thin Line map, you can take care of all but one of the first wave, set up layers of barrels to prevent surprise attacks; if object detection is off, alternate between going diagonally up and diagonally down to slide through zombies until reach a good point from which to take out your shotgun and Rambo em. Else just destroy the outer layer of barrels and Rocket/shotgun them from vantage point. Mine layers also useful, as you could for instance st up barrels in a design such that only one zombie can pass through, and they will have to be pushed through the path by other zombies (block off another layer between this path and yourself tho or this will fail) and put a patch of mines at the exit, so that the zombie, upon being pushed through the barrels by a mass of its kin, will obliterate that entire mass. Anyways, really just play it safe and don't rush it, though unlocking Infinite Range Uzi is vital for certain ploys.
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i was putting barrels and suddenly i got stuck in the middle i had to get out and what do i do i explode 20 barrels right beside me of course i died there isnt a worst way to die
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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
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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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edwardhowe method is stupid. my method works without messing with fake walls, and you get it done in less than 5 minutes! (plus you get to shoot lots of rockets)
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SUPER FAST EASY BADGE: simply choose nightmare, turn off all three options (devils, collisions, friendly fire) choose BIG BOXY level, clean out all the barrels, get a rocket launcher, stand in middle, fire up,up,down,down, you will be on level 40 in just a few minutes, then you will be on 42 in a few moments more! its so easy!!!