As noted in the other solutions, this must be done in the Hunter game mode, while you are playing as a C.E.L.L. operative. I found the easiest way was to grab a throwable object (preferably a shield), find a bottleneck on the map where you can crouch with walls behind you and to the sides, and wait. When a Hunter finds you, it will have to appear in front of you, and you can kill them instantly by throwing the object. You can then pick up the object again and wait for the next Hunter. If you pull this trick off twice in the same game, you can also unlock:
The Odd Job achievement in Crysis 3 worth 88 pointsGet two kills with one throw-able object in the same match (Public Match only)
Do this 10 times across a number of games to unlock the achievement. I hope this tip helps!