This acheivement will unlock when you finish level 11 and enter the tram to level 12.
*MAJOR SPOILERS*
During this entire level, the alien never shows, so in theory no need to creep around. That said, this level is full of rogue androids, so you still need to be careful (and one run in with humans).
Playing on hard, I could never get round them to hit them from behind as they always heard me, so I found the best route was to use the stun baton as they approach, then go mental with the maintenance wrench. If 2 come at you, lay an EMP mine, then use the wrench, or 2 shotgun shots to the head.
The only problem I had was when you use the lift initially, and come to the bit where the only way to go is blocked by the door with humans behind. Here you need to walk up to the door and quite literally touch it to trigger the mission to open the door. The humans see you but can't shoot you so don't worry. Start the maintenance droid behind you and then follow it into its "home". If you can't get the icon to start it (as I initially had) keep bumping into the blocked door and eventually the next mission will trigger allowing you to start it.
Good luck!!