Halo: Reach - Cross Mappin' Achievement

Noble 1: Kill a player at long range with the DMR in a matchmade Slayer game.

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This achievement can only be unlocked in Matchmaking within any competitive playlist that contains the Slayer game type. This achievement also requires the DMR weapon. The DMR weapon is usually the default weapon in Slayer, but can be found on any map. To unlock this achievement, you must kill an enemy using the DMR at a long range. You can tell if you're within the range when the reticule isn't red, but you're targeting the enemy. The kill doesn't need to be a headshot in order to count. I recommed you time your shots, and wait for the bloom to slow to the middle to be more accurate depending on the range. The easiest playlist for this achievement is Big Team Battle, as there are more enemies, and you play on the bigger maps. This achievement can be unlocked on any map

Easy to do, and nearly impossible not to get if you play Multiplayer.