This should more than likely be the second, if not first trophy you earn in this game.
Throughout the game you'll encounter enemies who will hide behind long wooden shields (pretty much doors), which have a slight hole in them at head height.
You'll come across one before moving into the barn for the horses on the first mission, but either way. When you encounter one, just move away from the shielder as you watch him, at a certain distance away he'll begin to follow you with the shield, which when he walks his head will appear through the hole, so just shoot his head and the trophy will be yours if he does.
You can use concentration mode to help you better your aim/give you more time for a better shot.