From Halopedia, the Halo wiki
Cross-Mappin' is an achievement for Halo: Reach multiplayer matchmaking added with the Noble Map Pack. The achievement is earned by getting a kill at long range with the DMR in a Slayer game. It is represented by a stylized DMR scope.
To get this achievement, the player has to be playing a Slayer game and using a DMR. The reticule has to be blue/grey, not red. Some easy ways to get it is Big Team Battle on Spire (by standing on the spire's top), Hemmorhage or Boneyard. Destroying a vehicle when the reticule is blue/grey also counts.
Another possible way is on Paradiso, where the player can stand on the highest point of the map (above the Forerunner tunnel) and kill someone at one of the bases.