"Zombicidal Maniac" and "What Are You Trying To Prove" achievements hint
It is possible for a friend to invite you into the last chapter of any episode (as long as it is from the Dashboard). Completing it from that chapter will make the game flag that you have gone through the entire campaign and not just that last chapter. This also works for all other achievements related to doing a "whole campaign", such as "Safety First" and "Upset Stomach". Note: Only people invited in from the Dashboard will receive the achievements, and not the person inviting.