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Climb or use Blink to get above a hostile. Drop onto them from a height. Make sure that they don't see you and strike when the indicator flashes on the hud.

Some good places to do drop assassinations:
Mission 2 or later - Outside Granny Rags' house, the Old Distillery District, a building on Clavering Blvd just after the first wall of light.
Mission 4 - Around and on the Kaldwin Bridge
Mission 6 - At the top of the water lock
Mission 7 - Just after entering Daud's base
Mission 9 - Kingsparrow Port