You need to stealth take-down 10 armoured enemies with melee by removing their helmets. Oh, and without being detected. You can unlock this trophy in chapter 8 during the helicopter encounter.
When you reach the helicopter encounter at the end of the chapter, head along the right side of the area where there’ll be a truck with a lockbox inside. Hide in the long grass around here and wait for the armoured rebel to walk by. Press square to grab him, followed by repeatedly tapping triangle to remove his helmet, and then press square again to silently kill him.
Reload this checkpoint and repeat the attack until the trophy pops.