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.