Step 1. Get a team of skilled players with quick reaction times. You will want to have played with them before to understand their playing style. Everyone should have a mic. This will mean the difference between failure and success. can be done with 3 bots under right wing of rescue plane on Dead Air
Step 2. Play a campaign on Easy, start at level 1. Play through the campaign until you get to level 5. You can take as much damage as you want from special infected. You will still get the achievement.
Step 3. Once the level 5 map loads and you are all in the safe room, call a vote to return to the lobby and have everyone accept the vote.
Step 4. Once you return to the lobby, start back on level 5. This will clear the slate of Special Infected damange and allow you to attempt the achievement from this point forward.
Step 5. If you successfully avoid damage from this point on by special infected you will all 4 get the achievement.
Step 6. If you fail, you will have to return to step 1 and try again. You cannot just go back to the lobby and start level 5 again.