Walk over to get the sniper rifle that the agency dropped off for you. There is a building across from here that has stairs that only go up one story. Make sure you do not get spotted with the rifle in your hand and make you way to those steps, but do not go up them. Use them to hide behind. Set your aim to where you are still behind the stairs and have the last bit of road that the limousine will get to before entering the castle gates in view. This should leave you with a good shot at your target. Once you have scored the shot, drop the rifle and walk calmly to the exit. It may take a few attempts to hit the target. Note: Kneeling helps.