at the first you need two controller with two profile signed in. then choose the map: raven down, cause its so small and only two objective. then set the mach on 120 point.
after that, the player who want to get this achievement, should get 2 caps and leave the other player alone.
infact you need to get 120 point and dont let the oppsite player get any point.
(NOTE: if your going to get THIS IS ANNEX! as well, you should set the mach on 3 rounds, so it will count for that achievement too)
hopefully this help