This can be achieved in the mission HIGH OVERSEER CAMPBELL.
Before you start the mission, a lady (Callista) will tell you about her uncle and ask you to protect him.
Once you get into Campbells meeting room, you'll find two glasses on the table. One is poisoned (that one is for Curnow) and the other (Campbell's) is not. Just switch the glasses and hide. Then the two men will enter the room, have a drink, Campbell dies, Curnow escapes. The achievement will unlock once Curnow is gone.
Alternatively, use sleep darts, and hide the unconscious Curnow somewhere safe. In that case the achievement should unlock at the end of the mission. This way you avoid killing Campbell.