To get this achievement you need to progress with 'The Witness' Side mission.
When you get down to the first med station outside of the Police department, a guy will give you some materials to upgrade your gear. When you are done upgrading he will disappear for awhile. You can find him again after you get the EMP Starfish drone. When you have the EMP Drone, head back to the first healing station where you found him originally. Work your way back to the river and go to where the guy is standing in front of the flaming barrel. If you look at the railing along the road, you can see one piece is missing where you can drop off. Drop off and fall below, killing the punk down there. Use the EMP to open the door down there and enter into the underground, inside you will find the hunter. He will ask you to bring him some AID gear, which you will have to craft yourself. Kill the enemies at the security checkpoints for the gear. Sell it to him, he will head off, and you can loot the chest. This isn’t the last time we will see him.
Progress through the story until you get to A.I.D. Command, as you progress through the level and climb the final tower to get the Force Hook, he will be at a locked door and say he needs you to find a keycard to open it up. You should have already looted the key. Talk to him and tell him you have the key and then open the door. You will get to find out what happened and should unlock the achievement.