This achievement is missable because there are no enemies after the story, and it only works in certain story sections. Serpent's Fury is a skill (green) that can be bought at base camps. First, buy "Serpent's Strike" (one skill point), and then "Serpent's Fury" (two skill points). To perform the move, you must stealth kill two enemies standing next to each other. Quickly press Y (Xbox controller) repeatedly and Lara will jump from the first enemy to the second, killing both of them. It is fine if other enemies spot you doing this, just to initiate the move you have to be unseen. Repeat this on three pairs of enemies to get the "Chain Gang" achievement.