Location: Diaz can be found at Shackletons Peak.
You have to get the magnetic hook before you can get this achievement! You will automatically get it after finishing the main missions for the Camp of the Forgotten (the northern area on the world map). Once you get the magnetic hook you must go back to Shackletons Peak. Activate the console at the bottom of the mountain to call the gondola. Use the gondola to get to the summit immediately. You will have to do some backtracking now. Go through the door and down the mountain where you previously came from when you climbed the mountain for the first time. Go until you find a point to use your magnetic hook. Its just a minute walk away from the summit (on the left side of the path). Use your hook and keep going until you reach a hidden cave. In that cave is the Lost Contractor. Pick up his audio log to unlock the achievement.