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.
