I've created an achievement guide on my YouTube channel covering this. You can get this while playing through Chapter 4: Part 5, Protector's Duty: Obligitory Cooperation. If you missed this for any reason you can simply load this from the chapter select on the main menu. This video shows the fastest way to play through the puzzles on the docks and get to the part with the guard that you need to spare.
If you don't need to see how to solve the entire puzzle and want to skip to the part of the video showing the requirements for the achievement skip to roughly the 5:00 mark.