This achievement will come by killing the Vorus Centurion. He is the first boss you will encounter. I will be giving tips on how to defeat him on Brutal difficulty, although you can use the same tactic on easier difficulties.
From the starting point move down the first flight of steps and shoot the explosive canister on the right hand side. That is the wall you will be keeping between you and the big ass gun he wields. Make sure your back is up against the wall! Do not worry about looking around for ammo, you will die. Your pistol has infinite ammo, put it to use!
1. From here you should see his left arm (your right) poking out, Keep firing at his arm and don't let up.
2. When he nears 3/4 health a reinforcement will arrive of just 1 enemy. Take him down and repeat step 1.
3. Again after a while he will spawn another guy who will be on the same walkway to your right. Now you will notice he is shooting these rockets at you. To avoid this move to the next pillar (towards where the second reinforcement arrived) and put a few bullets in him.
4. Repeat the above process going between the 2 pillars and taking a few shots at him when he stops firing at you. By thhis time you should have him beat.
I will not lie though it can be frustrating as sometimes when you spawn he can kill you almost immediatly, but keep perservering and you will take him down. Good luck!