Here is my video contribution. This guide in particular shows you how you can get this no matter where you are in the game/what equipment by doing it in the first level.
Hunter Class is the best to choose as you'll find a sniper, but can be done in 5+ mins with any class.
Hope this helps!
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