Can be done on the first story mission on Mars. You'll get to a point where your Ghost will open a massive door that will introduce you for the first time to the cabal.
If found I could snipe the cabal with a Sniper Rifle & Hand-cannon for easy headshots in the same corridor. Just lure them foreword and poke your head out briefly if you need them back.
They have relatively large heads so they shouldn't be a problem to hit, but some will be using shields. For them just shoot their arm, and they will stagger, revealing their head for 2-3 seconds.
Pick the first difficulty option if the enemies are hard to kill, and try doing this at about lvl 16 at minimum.
Hope this helps guys!
Also, here's a handy video guide I made
Also; for those having trouble unlocking glitchy achievement do this:
Press the Menu button RB over to settings, highlight Log Out and choose Switch Profile (not Change Character). This will take you back to the game start screen, and once you press A, it will ask you to choose your Xbox Live profile.
Choose your profile and then choose your character from the character select screen. Any achievements that should have been unlocked will then unlock.
*Cheers to FLEEBS & ludo10109 for the info