Mission: Get the tank of water.
When you exit the cave with damir towards getting the tank of water. You'll see a chain going upwards towards a platform. Sneak round the right hand wall from where you entered and wait for the guard to walk away. You can scare the slave, he just gets redirected to the level above. Crouchwalk all the way to the chain and hold Square next to it. At the top, the trophy unlocks.