Kill 2 rappelling enemies in a row before they land on their feet in Single Player or Special Ops.
You can get this in campaign, in missions like Whiskey Hotel or The Only Easy Day... Was Yesterday, but you can do it in Spec Ops mission called Suspension in Alpha. Take a few steps, and from both sides of the bridge there will be a squad descending. Kill two of them, and achievement unlocked. Here's a video by Mistical:
Another good mission to get this on is the Special Ops mission Overwatch. If you are the player in the AC-130, you can shoot down enemy helicopters easily with the 40 mm weapon. When you see an enemy helicopter, wait until there are a couple enemies dropping out of it. Once you see them, shoot the helicopter down. All of the rappelling enemies will drop down and die and you will get the achievement.
Do this on the "Suspension" level on Alpha in the Special Ops mode. Within the first 20 seconds of the level, you can get this achievement by shooting the rapelling terrorists on either side of the bridge.
I completed this on the Campaign during the level "Exodus". During part of the level, several enemies to your left rappel out of a helicopter. It's easy to shoot both enemies; they are right next to each other.
In the optional mission "No Russian", you can easily line up a shot just after the group descends the escalator (just before they are about to engage with the riot cops). In the airport background there is a helicopter hovering in position with troops dropping out of it. Line up the shot and just keep firing--you don't even have to see them because they drop straight into your gunfire. If you happen to mess up, you can reload by simply shooting the terrorist, placing you back just before this position.