How to Acquire
A way to reach the top is to either use Blink or Far Reach to climb each successive roof, using chimneys to gain more height. Another way is to climb up to the top of the elevator shaft and use the open window to reach the roof above.
The easiest guard to reach for the drop from that roof is right below along the facade, facing the docks on the other side of the building. Interestingly, killing a Doppelgänger also counts towards this achievement, so summoning one and killing that is also very easy.
Note that this achievement cannot be completed without powers.