:) make a session, with friends, it must done on shatter. Two Mantis's are present. The vid below shows how easy you can get the achievement, you've probably done it before but on a different map.
The vid is on valhalla, but its meant to be done on the crimson maps
The mantis on Shatter is located at each spawn below the bridge (overpass).
This achievement is linked to David and Goliath