Complete 'Of Their Own Accord', 'Second Sun', and 'Whiskey Hotel' on Veteran difficulty.

Once you clear the mezzanine in the main room of the first building, you'll hang a left into a hallway with far too many entry points. Be careful here as the baddies can pretty much one-shot you through the door straight ahead while you're taking care of the ones around the corner to the left, and vice-versa. At the crow's nest, pay strict attention to Foley. I know sometimes the AI's suggestions are kinda wacky, but this part is pretty much dead on. Also, when you hear "get your ass to the roof," run like hell.



Achievement is worth 25 gamerscore.