Campaign: Swamp Fever
Level 2: The Swamp
This is a lot easier than it seems. Once you get to the plane, open the door and huddle between the seats either to the right of the door or as the videos shows to the right. The AI will kill off most, if not all of the Infected so you can sit back and wait.
The AI can take damage but you must not take any, this is the same for other players. Anyone who wants to earn the achievement must not take any damage.
Check your health before you start this as if it becomes lower at any point, you will need to start over.