This achievement must be accomplished through ranked matches. Player matches DO NOT count towards this achievement.
For boosting purposes I have found Training Facility (because it is small) or Frozen Wasteland (because of the tall tower in the middle) best for quickly spotting your boosting partner(s) and acquiring kills.
This achievement took me and my partner about 2 hours to do of 20 (total) matches of elimination with the first match 10,000 battle gauge and the successive matches after wards 2,500 (with an accidental 5,000 or two throw in because I forgot to change the battle guard when setting up).
This achievement could be accomplished while working on your elimination/team elimination medal achievement.
This achievement will unlock one you have equaled or passed 50 career headshots and after a match is over.