Medal of Honor Warfighter

Everytime an enemy dies, he'll drop his weapon, just scoop them up and use them on the next batch of enemies. There's a few different ones, the shotgun is easily the hardest to use (at times), but whatever weapon works for you is good. It should only take you a level or two to earn this if you continue to gathering weapons from corpses.

NOTE: Changing weapons (triangle) DROPS the enemy weapons. You can pick them up again but they are NOT a permanent part of your arsenal.