You can just do the glitch for Nothing special, play a campaign with you and 3 friends until the 5th chapter, don't touch the bar on the door, call a vote go back to the lobby and then restart the 5th level of the same campaign and just don't get hit by a special infected. Works with basically every achievement. It's how I got it.
Crap bro. Hey if you want, I'll help you with it possibly today or tomorrow depending on what I'm doing. If you can help me with the last Expert campaign I need. SoiWilliamSoi. I'll get the team together who helped me get it.