If you do choose to play though the story mode to get this achievement there will be a few freebees. On each playthrough you will encounter several technicians and scientists, i'd guesstimate at twenty or so. They won't attack, they go into the fetal position or kneel with there hands up. You can crouch and run behind them. Press B to stealth kill will be an option.
I got my Shadow Hunter while killing a guy with this method.