Here is a quick video showing how I unlocked it:
From my description:
""Deal 5000 damage in one shot."
This is how I unlocked it; by playing as Commando, on Rainstorm.
After defeating the Ancient Wisp, it spawned a bunch of smaller enemies. Attacking all of these at once with Full Metal Jacket (a piercing shot that looks like 2 orange lines being broken by a white one) resulted in me dealing over 5000 damage, and thus unlocking the achievement. Mostly it's just luck, but you're bound to get this the longer you play. (As difficulty scales up and you deal more damage per shot.)
It's not necessarily difficult to unlock, but does require quite a bit of luck. This was on my second run I believe, so this could be one of the first achievements you unlock! (As it was for me.)