To get this achievement simply load up the mission 'Tip of the Spear' on Normal or higher difficulty (Rally Point Bravo is quicker than traversing through the level).
After you have got the Rocket Hog and ventured across the bridge with barricades, you will hear through your comms about an Elite Zealot. This is the one we are looking for.
Make your way through the building/complex until you come to an open area with a plasma turrent opposing you. You will find the Elite Zealot there. Killing him will earn you the achievement.
I made this video to help:
Hope this helps.