The kill can be a headshot OR bodyshot with the DMR.
You must be far enough that the reticle is NOT red but blue while you are zoomed in shooting your enemy and that sometimes doesn't even suffice.
You do NOT have to get this on one of the Noble Map Pack dlc maps (but you can).
The gametype must only be a FORM of slayer and doesn't have to be in the slayer playlist (but can be).
|> Load up BTB and vote for big team slayer on HEMORRHAGE, PARADISO, BONEYARD or SPIRE. (Shoot from one end of the map to another on boneyard - on hemorrhage shoot from the top of the inclined hill behind the base or if you spawn at the other base use a jetpack to get up onto the top of the cliff near the coastline [yes you can get up there!] - on spire you should clearly shoot down from the top of the spire - on paradiso shoot down from the top of the rocky peak in the middle of the map)
[[[ OR ]]]
|> Load up the DLC playlist and vote for slayer gametypes on BREAKPOINT or TEMPEST. (If you end up on tempest go to the very edge of the map, on breakpoint stay on the top of the ridge in the middle of the map of course. The latter is where I got it while shooting an enemy behind the banshee spawn) :D
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This is not attainable in the Invasion SLAYER playlist
This should be attainable in the S.W.A.T. playlist
This may or may not be attainable using the TELEPORTER tactic in which you nail a kill and then quickly step through a teleporter which will take you far enough away, the idea being it records the distance after you have teleported rather than instantly as you pull off the kill [?] (please confirm)
Thanks go to 'jtusa425', 'Kol Kavs' and 'Pagemaster146' for the developments.
So; if you're having trouble with my prior suggestions perhaps one of these above^ alternatives will do it for you :)
-So you think you're far enough? You're NOT. Go FURTHER. [or try a larger map] Alternatively; jetpack as high as possible when administering the final headshot to add some crucial distance.
-If your distance is fine but you cant pull off the shots fast enough to get the kill try a sniper bodyshot quickly followed up by a DMR headshot as shown in the video below =)
Hope this helps
All credit for this youtube video demo goes to Zawk15.