There's a few ways to do this.. If you can't find him, you can always use the directional microphone to identify which direction. When you do find him walk slowly with the d-pad and creep up to him, then when you're behind him hold square and it'll start a hold up where he lies down.
If you fail, and he sees you and shoots you, you can chase after him, sometimes he'll run out of breath and you can just walk up behind him and hold him up that way.
If he doesn't and he loses you, you can use thermal goggles to trail him. NOTE: if you walk up with the left stick behind him he'll likely hear you so go slowly around corners and again creep up behind him.
Once you have him in a hold up, he'll lay down on the ground... Point the gun at his head and then his body 3 times to make him drop his uniform and cause the trophy to unlock. The first two he'll say "I don't think so". You only have to hold him up once to get the uniform and the trophy, it just takes a few seconds to get him to give up his loot, unlike most soldiers.