Left 4 Dead

The best location I found for this was in chapter 3 of the No Mercy campaign. Once they leave the spawn, get through the alleyway, go through the fast-food place, and end up in the large lot with several trucks, they will head for a lift. Typically speaking (almost EVERY time) they will wait for all four survivors to be on the lift before they activate it.

If you spawn as a boomer, head up to the rooftops from the very start. Get up to the HIGHEST point you can, and head over to the wall against where the lift is. Hide until you see all the survivors get on the lift. Once they are all on, drop down onto the next roof, move forward a bit and look down, and click that right trigger! You should be able to get all 4 every time, just make sure to move your aim downwards or upwards, as it should be a pretty straight line.

Check out this video (not mine; full credit goes to its maker!):

To get this achievement I hooked up another controler and played offline verses. Make sure the person that needs the achievement is the infected. I did this one no mercey. As soon as the first stage starts all you have to do is move the 2nd player in front of the ammo table. The Computer will follow. The regular infected will open the door if you wait. Spawn right out side the door. Then walk out and puke. I got it first time. Its not to hard.

Set up a private match and have your friend join humans, you be infected... Eventually the infected player will have the option to become a boomer. Have the human player lure all his teammates into a room or behind a wall, then shoot all his teammates and leave them in a neat pile by a corner or easily approachable location for your boomer to creep up on. Then, jump in close and spray your bile, cheevo unlocked!

nb. It can help to have the human player stand between the fallen survivors and the boomer to protect from pistol fire from downed survivors.

Easily Done on No Mercy. On the hospital level in the beginning don't respawn just walk around and go through the level. Get to the elevator and walk through it. Climb up the ladder all the way to the top. When you get up there break throught the vent. When you step outside you'll see a large yellow cart. Constanly hit it until it is blocking the door to get off. By the time you ar e finished the humans should be in or very closer the the elevator. When they arrive they will atempt to rush out and be stuck quickly jump out and spray them.
Good Luck.