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For getting the achivement select "nightmare" difficulty, object collision, friendly fire and devils "off", select "Boxy" map, block one of the entries with "fake walls" and then kill everything with "rocket luncher". At the end of every level get everything zombies have left off, sure you'll recharge your rockets. Sure "win". Press "+" for keeping this comment "alive".
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The secret of Life dear boys and girls... is explosions. Specifically grenade explosions. All you need to succeed is to spam grenades and have friendly fire turned off whilst in Nightmare mode. Took me less then no time.
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the game is s bad because of the MOVEMENT you want to walk left then turn around you will keep going left. +if this is a problem for you.
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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.