You will earn a medal once you earn 2 or more enemies rapidly after one another with any of the specialists WEAPONS. Here are some examples:
Seraph -> Annihilator (6 round instakill pistol)
Battery -> War machine (6 round grenade launcher)
Firebreak -> Flamethrower
Prophet -> Tempest (electricity shot, one to down enemies, another one to kill)
I personally got this with seraph, I really like the annihilator pistol, even at long range it takes down enemies instantly. You will need to get a feeling of when to pull out the gun; which is just before you get into combat. While you have the weapon out it will deplete. It will also deplete by firing shots, 6 at maximum. I would advise not to play in hardcore for this as the weapons are instakills anyway, thus you can better have some extra health yourself.