You must be the one on the rope, it doesn't mean to kill other enemies that are suspended from ropes. This can be unlocked through a combination of campaign, terrorist hunt and adversarial... the first two being the easiest. When playing through the story mode you may find some places that you notice would be good for this achievement. Otherwise, a good map for doing it in terrorist hunt is Presidio. From the start, go to your left and on the rappel rope between two doors. Look back and forth between them and fire your gun a bit and enemies will start pouring out. Try to kill five or so each time and you will then likely die. Just restart the level and repeat until you get the achievement. Some other maps with rappel ropes are: CQB Training, Kill House and Streets.