This achievement doesn't really need a guide to be completely honest, but this is more of a heads up than anything.
You will come across a mission in the Western Precinct called "Ghosts", which requires you to kill a bunch of Covert Op soldiers along a path. When following this path, just run around with your Thermal Vision goggles (equipped by pressing
twice), and when you find them. Sneak up behind them and stab 'em in the back.