Cross-Mappin'
From Halopedia, the Halo wiki
Cross-Mappin' is an achievement for Halo: Reach multiplayer matchmaking that will be made available for DLC playlists with the release of the Noble Map Pack. The achievement is earned by getting a kill at long range with the DMR It is represented by a stylized DMR scope.[1]
Many players online have found that this achievement has been quite difficult to get because of the vagueness of the achievments description but has been thought to be literally doing the achievements name, that is, getting a kill with a DMR from the length of the map that you are playing on.
Tactics
One possible, and easy way of doing this is to play an invasion game on Spire and stand at the top of the Spire, shooting players from there with the DMR.
To get the Cross Mappin' achievment you have to be playing a slayer game and using a DMR, also the recticle has to be blue/grey it can not be red, some easy ways to get it is Big Team Battle on Spire, Hemmorhage or Boneyard. You can also shoot a vehicle as long as recticle is blue/grey for this is a larger target and if damaged can be an easy kill.
Another possible way is on Paradiso, where you stand on the highest point of the map (above the Forerunner tunnel) and kill someone at one of the bases.
