This is an unmissable story related achievement. If you don't want spoilers just play, you will get it.
More detail and strategy in the spoiler tag.
*** Spoiler - click to reveal ***Towards the end of chapter 6, Joseph will climb onto a building to cover you with a sniper rifle. As you progress you'll come across a large enemy, after you kill it another will attack. Once both are dead the achievement pops. I found the easiest way to kill them is explosive bolts from your crossbow. After 2-3 bolts they will fall to their knees, at this point just use your shotgun/revolver until they die. Freeze and shock bolts are also effective if you have plenty of shotgun ammo. you should try to lead them back to where you came in so Joseph can put some damage on them. Failing this you can run past them, there is more ammo and a place to hide in a small courtyard to the left.