Left 4 Dead 2

My Tutorial on how to get this as well as Scattering Ram

This method requires two xbox 360 controllers, OR an xbox live friend to participate.

Attach a second controller and set up their profile on L4D2.
=Create a private lobby for only you and the second controller, I recommend setting up a scavenge match as it will take almost no time doing it this way.
=Wait until the intended profile you want the achievement on is the infected. Make the second controller person leave the glowing area and start the game and place the character in a corner or small area [this way the AI will follow and all be in a small proximity].
=Now go back to playing as the infected and wait until you get the spitter. If you do not get her throw yourself into the human AI and they will kill you. As only 2 people are playing it will take only a matter of seconds to respawn.
=If however, you get a charger, you can also get the 'scattering ram achievement' by aligning yourself carefully and charging into the humans.
=When you get a spitter hide behind a pillar or tree and aim at the legs of the group. You should cover all survivours and get the achievement. You can tell if you hit them all or not by the damage multiplier going crazy and showing [gamertag] hit [gamertag] x10 or something similar, and this appearing for each character.

If it does not work the first time, rinse and repeat. I used this solution and got the achievement no problem.

Edited 25/7/2011 to remove spelling errors.

Pretty easy to get this achievement, you just need to wait for the right moment.

You don't need to spit on everyone all at the exact same time, you just need to have all of them take damage from it simultaneously.

For example: The way I got it was, waited until they stepped into a trailer, spit in the trailer and hit 3 of them, then a moment later, the 4th survivor stepped into the acid as the others were still in it. *boo-doop!* Achievement unlocked.

Be patient and scout ahead of them to where healthpacks are and you know they will have to be close together (i.e. hallways, elevators, etc...) The elevator in the Hard Rain map after the cornfield is a perfect spot to get it.

Survivors have a tendancy to be just far enough away from each other in open area for you to miss out on this achievement. Great spots are tents where they are in a confined space, like the CEDA tent uphill from the saferoom start in Dead Center Part 2.

If they've been aggressively atttacked to this point they will use this to heal up or might check it for supplies. Just hide around the corner of the tent until all four enter. A well placed spit in the center of the doorway can unlock it.

If all four don't enter, it's ok. There is always the room they have to go through down the path from the tent, just follow behind and maybe wait until they get down the stairs.

Panic Events in general are a great opportunity for group-spits, but the airplane crescendo in Swamp Fever is a particularly good one. Spawn on the wing, and if the survivors hole up in between the seats, blast the unsuspecting suckers through the windows.

You don't need to do it there, though; if you play Versus enough, you'll get it eventually.

You can get the achievement split/ screen in the finale at the mall. The one going for the achievement choose infected and the other player go to the stairs and make sure the AI are close to you. Spawn and shoot from the opposite side so the AI won't see you and run away and start shooting at you.

The same principle as the Barf Bagged achievement in the first Left 4 Dead. Just wait until the survivors are in a small group and spit. Get up high to open a wider range as spit and barf go down.

Okay for this you will need 2 controllers, go into scavenge mode (Dark Carnival, Motel) and have one person as the infected and have the other set to survivors. When you, player 1, have control of the spitter in your round quickly grab your other controller and move everyone into the middle of the map, then sneak carefully up behind them and pull the right trigger, it's as easy as that :)

You can also grab the "scattering ram" achievement using this method as well.