For this achievement, you must have a total of 170+ Unique Human kills in the game. Those are "different" humans. This means you cannot just reload checkpoints and kill the same people afterwards. They all must be different people. The best way to tackle this is to stay in an area where you can set off alarms and keep triggering them and killing the enemies. You can check your stats from the main menu for "current game" kills. Once you have hit 170 or more, then you will be good to finish the game and gain the achievement.