This achievement is extremely frustrating even when playing on the easy difficulty.
The description says that all survivors must complete a campaign without being vomited on. However, it neglects to tell you that no survivors can be exploded on either. So you will need to complete a campaign without coming in contact with boomer bile at all. Only shoot them when you and your team are not in close proximity. If one does catch you off guard, remeber to melee, this will knock boomers off balance and give you more reaction time. Also, all four survivors must finish. You can't kill the computer players and finish by yourself. If your luck runs out and someone gets covered in spew, you can't restart from the last checkpoint, you have to exit to the main menu and start the campaign again.
I found Blood Harvest to be the best campaign to complete this. I also played with computer controlled teammates as they will be able to take out boomers hiding in the brush before you can even see them. Another trick is to turn on full captioning in the options menu. Now, during gameplay a message box will appear on your screen showing the dialogue, and more importantly, special infected. These messages will warn you of boomers long before their sounds become audible.
During mayhem scenes when you alert the horde, or just during infected attacks, simply melee and let your teammates kill the zombies while you keep an eye out for boomers. At the finale, i stayed to the left of the front door of the house you call for rescue in. Here you are a good distance away from the action, and will be able to pick off boomers before their throw up becomes a problem. Again, let your teammates battle the infected while you make boomers your top priority.