This achievement is similar to the PS3 platinum trophy. Just complete all other achievement like it says and it will unlock.
Some of the achievement can be tricky to get so if you get stuck just check out some of the guides. I would say hardest achievements are the collectibles because they take so long to collect.
UpdateIf your having problems with it unlock try there two fixes some member have posted.
Posted by xD RoDD
I install the game and load the last chapter, the last mission, when i killed the first guy it poped for me
Posted by FarmerJakMehauf
I just replayed the last mission and got it as soon as i killed a guy. i didnt install the game or even restart the game. just when to mission select after finishing the game on 1% mode.