This achievement can be gotten during the third mission. While exploring Addermire, you'll eventually work your way up to Hypatia's office. From there, head outside and use Blink to work your way up to the roof of the area next to her office. Blink to the chimneys, then use Blink to get back to the roof above Hypatia's office. There is a bone charm on the roof above there you can use to orient.
The entire roof above her office is considered the highest point of Addermire, so you can do your drop from anywhere on the roof. If you head toward the back of the building (left from the direction you likely got onto it), you will see a patio way below which should have some guards patrolling on it. Save and then try to do your drop kill. Reload and repeat until successful. You want to find a guard actually moving about midway between the building and the wall. A guard leaning against the wall looking out to the water is probably out of your range.