To get this achievement you need to unlock all the other 34 achievements.
Some say that it is bugged and cannot be unlocked, but note that some of the achievements are hidden and show up only when you get them, so even though it may look like you have all the achievements from the list but one, the hidden ones still prevent you from getting Achievement Whore.
If bugged, can be unlocked by starting up the beta version of the game. Just replay the prologue chapter and it will pop-out as any other achievement. To enter the beta simply right-click the game title in Steam, select Properties, and then go to the Betas tab to opt-in. Password is: hlm.