Seeing as I am one of the few people on Neoseeker that have all the achievements on Left 4 Dead, I thought I might share my tips on achieving some of those more difficult achievements.
Nothing Special/Stomach Upset/Do Not Disturb: This will require at least two people to accomplish these achievements this particular way. First, get one guy to start up a game on the Dead Air finale. Everyone else that is planning to help must be at the 360 Dashboard. The guy that started the game sends invites to them all. Do not open the safe room door until everyone is in the game. Now basically from here all you have to do is go untouchables on the special infected. I found the easiest way to do this by beating down the turret until you are able to jump on it where it will bounce you up onto the escape plane. Make sure to kill off any AI this way as they will be hurt by the special infected. Everyone on the plane can just beat each other down so that if a hunter or smoker gets them they get off immediatley. Tanks will bleed out but may throw a rock so watch out. Once the plane is full run down the back of the plane by the tail and jump in.
The other way would be going back to the far corner and kill the zombies as they come to you. When the tank spawns unload on him, since the area is so open he should drop very quickly. You may also shoot the rock when he throws one just in case it were to hit a survivor.
If no one gets hurt from special infected the three that came from the dashboard will get the achievement, so remember to do it for the other guy as well. For some reason if you come from the dashboard it counts as you having played an entire game.
Untouchables: I found this easiest with the three AI on Dead Air. If you go to the back corner under the little building you should have a great shot at getting this. Since the area is wide open the AI can easily pick off zombies and they can find the special infected very fast. I also recommend using the pistols during this part to save ammo. Now when the tank comes the area is perfect for taking him down in about 10 seconds. If he throws a rock try to dodge it but shoot it at the same time in case it is headed straight for a AI. Right after the second tank is killed rush to the plane. If anyone has a pipe bomb or Molotov now would be the best time to use them as it keeps zombies away that much longer for you to get to the helicopter.
Zombie Genocidist: This one is more of a leave the game on while you do other things or sleep kind of thing. Start the No Mercy Finale. Kill of the three AI and the grab a pipe bomb and head up the stairs. Chunk the pipe bomb down the hallway and the run back to the safe room. After some time a boomer should could and puke on you, calling in a horde of zombies. This is where you stand a few feet back from the door, put a rubber band around the meele button and your set. The boomer should puke non-stop so the zombies will just keep coming. You should rack up over half of the 53,595 kills in one good nights rest.
CrOwned When attempting to Crown the witch, I suggest using a auto shotty. Take your time and move up behind her with the flashlight off, when she is starting to get mad and standing almost all of the way up, blast her right at the hairline. Shoot more than once because if you don't headshot her she will most likely incapacitate you. It may take a few tries but it will become easier after a few failures as you will realize your mistakes.
Well I hope that helps you. And I would like to thank many of my friends for showing me these as well as some people from over at TA(TrueAchievements).
TX, 
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