To unlock this achievement you need to finish a level with exactly 47 kills. A good mission to obtain this achievement is 'The Meat King's Party', as there are in excess of 60 people throughout this mission. Everyone that you can kill, including civilians, police and enemies all count towards this, but targets DO NOT. You should keep a mental count of how many people you kill, as there is no counter during the mission. After you have killed 47 people, proceed to complete the mission, remembering that you cannot kill anyone else after killing your 47th victim.