In Breaching the Perimeter, you will eventually be given the objective "Get inside the Security Tower." Shortly after it is given to you, you will climb up to a rooftop with a bunch of patrolling guards. Once you get far enough, you'll see a guard standing under a breakable light, another guard watching him from the ledge high above him, and a door into the next area behind the first guard.
Do whatever you want with the first guard on the ground, taking care not to alert the guard up on the ledge in any way. Then make your way through the building to the other side where you can climb up to the same ledge the guard is on. Now kill both guards nearest you when you get up there, taking care to leave at least one body up on the ledge with you instead of stringing them up or dropping them off the ledge. Now simply drag this body over next to the guard and throw it next to him. If you hit him, he'll just notice the body and call in an alarm. If you make him just notice the sound, he'll freak out and fall to his death, netting you the "Unstable Footing" achievement.