As you progress through the game you will complete the Knife in the Dark
story mission and unlock the Death Threat
skill. Equip it and find a worm (marked by a green icon) and interrogate him to bring up the Nemesis System menu. Find an enemy and issue a Death Threat using Y. Issuing a death threat will cause the enemy to be a higher level but also drop higher level gear. After this, a new mission will show up on your map, and you'll need to complete it. The mission will be unique to the area and enemy. You'll need to follow the mission to lure the Nemesis out, and then take him down to earn the achievement and unlock gear.