Achievement text: "Successfully knock out Walton Simons."
To get this, you must KO Walton Simons with tranq darts. You can damage him any way you want before he goes down, just as long as it's the tranq dart that take him out.
There is a way to cheese Simons to make him an easy target. When you encounter him in the underwater lab, run past him to the flooded room with the ladder. There will be a scuba trooper there. Take him out if you want. If Simons is still chasing you, he will enter the room and jump in the water. However, due to a bug in his AI, he will not come up for air. After a minute or so, he will start drowning. Once he's taken enough damage, he'll switch to a DT sword. You can shoot him with whatever you want now. Once he cloaks, start save-scumming as you damage him further. Right when he's about to die, hit him with a tranq dart, and it'll finish him off.