Like the others have stated, this is done in Facility, near the beginning of the level.
You'll eventually reach the top area of several toilet cubicles, one of them having a man sitting there, reading a newspaper and whistling a tune. Get on top of that area and press X. Bond will slowly make his way towards him. The guard will finally look at him and be surprised. Press the left trigger when prompted and Bond will take him out in rapid succession, then roll and kick the door to get out of the cubicle. The achievement should pop up if done correctly!
To see how it is done, please check out the Youtube video below. All credit goes to iBuzz7S for making this video and having it shown.
Edit: Not exactly sure why someone would give a negative vote to my solution when I clearly explained everything and even showed a video of the achievement. If you think I did something wrong, let me know in the comments below and I'll do my best to make the correct changes. Thanks!