This is for completing the New Year's Eve mission at the end of Year of the Snake. Similar to Nightmare in North Point, this is not a point of no return, so you can continue cleaning up secondary content after completion.
The mission starts with an uneventful drive to the docks. Once you're in the boat, Teng pilots, and you're on boat destruction duty. You have infinite ammo, so go nuts. Aiming for the people might be faster, but body shots on the boats take them out in about half a clip.
After arriving, run up the stairs, take cover, and take out the handful of guards. When the game tells you to get into position, however, run around the marker to the opposite side where you landed. There's a cargo container next to some stairs going down into a hold; go into the hold to pick up an Evidence Bag and prevent yourself from having to come back for it later.
Now get into position and the guards come storming out. Prioritize the guys on the deck above you, then take them out as you please. Without the face meter, your health will regenerate up to 40%; take one out, reload and wait to regenerate, then repeat. Once that's done, you can arrest the suspect inside. The achievement pops after the cutscene.