Left 4 Dead 2

Easiest done in very first level in hotel. Try to stay ahead of the group and listen very carefully for the sound of a Spitter. This is a good area simply b/c once you hear it, you can round every corner and just fire repeatedly or rain down melee hits. If the horde rushes you, just stay back until they are done so you do not accidently trigger/kill a Spitter.

Edit: There are reports that you can also get this achievement in Versus mode w/ a friend. Simply boot it up, kill all of your AI teamates, and have your friend continue to spawn then be killed until it comes up that he will be the Spitter. Then have your friend let you kill him without him attacking.

This is the easiest way to achieve this...

a) Set up 2 controlled players in a game of versus and make players on different teams.(Player getting achievement should be human)

b) Start the game on The Parish's first level.(Make sure game is either private or friends only)

c) Now, with the 2nd controller, just keep jumping into the water until you are a Spitter, then respawn with it.

d) Move the spitter around a corner, and take the first controller and then just go shoot him.(Make sure you don't shoot with the spitter or you'll have to become a new Spitter)

e) You should unlock the achievement once you kill the Spitter.

Another method and the way I got it was by throwing a molotov outside the door of a room that we were held up in while a horde was coming. I heard a spitter so i threw the molly at the door and she died before she could spit, keep in mind she only has 50 more health than the boomer, shouldn't be too difficult.