There are most likely several opportunities for this one throughout the game, though I found mine on the last map, Dam.
There is an outpost in the center of the map, just to the east of your hideout called Gaivlisi Village. If you approach from your hideout on the western edge, you will be on a hill overlooking the town/outpost below.
The video below shows you the outpost that I'm talking about, though it doesn't give you a good view of the map. You'll know you're in the right place when the game tells you that you've reached Gaivlisi Village.
You can take the shot from the hill like they do in the video, or move lower down to the area below to get a better angle on shooting up, through the wall into the guard. What I ended up doing was shooting him in the leg through the wall, alerting him and having him duck behind the wall. Then just estimate where his head his and fire through the wall again.
This got me the trophy.
Credit: Video Game Intro's Endings and Trophy Guides