For this achievement you have to kill Djimon Keita. He was the 3rd boss I terminated after Wilhelm Berg and Reza Khan. To make him vulnerable you must destroy Chimera Fields.
Once he is vulnerable you can get to him at the very north of the map. You don't need to fight anyone, unlike the previous two (if you've done these already). You need to hack 5 valve controls and destroy the respective valves. Once you do this, he will be terminated, and the achievement is yours.