This is one of the easiest achievements on L4D2. In the campaign Swamp Fever there are only four chapters instead of five. You need to complete all the chapters in order from first to fourth/last in one sitting. You can't play the first two chapters and then quit and later play the two last chapters. It can be done on any difficulty and with anyone. Though you must survive and come to the boat at the end of the campaign otherwise you will not get the achievement.
Peace and happy gaming!