Urgh, this level. If you want to do this Silent assassin on Professional difficulty, you're in for a tough time.
Silent assassin rating
-Find the way to your contact. There is no reason to avoid the soldiers yet, you're allowed to run around if you're not open carrying.
-Before accepting the sniper rifle, you need a guard disguise. Choose a guard near the pick-up, strangle or sedate him and drag his body around the corner and take his clothes. Now pick up the sniper and ammo. Holster the sniper and stay well away from all guards as you move to the other side of the city.
-Note: it is possible to get to the sniper point in civilian clothes without being seen, but this is tricky as the guards will open fire even at longer distances. And even at the sniper point, the guards might see you climbing the stairs or sitting in waiting.
-There is a staircase leading to the top of the mosque, and a stairs+plank leading to a ladder of a three-floors building. Take position at either. Please note that the motercade will start to arrive the moment you get to either of these points, whether you have the sniper rifle or not. Once it reaches the other gate it's game over and any other weapon will not penetrate the limo glass.
-A save here is highly recommended if you have it. Now wait for the motorcade to approach. Choose a suitable spot and shoot the target in the limo. Try to hit him as soon as possible: the UN soldiers will open fire in retaliation and if they shoot a muslim soldier by mistake this will still count as an alert against you. And one tip: Not even an armor-piercing sniper rifle will go through one person into the next, so don't try shooting the target through the driver.
-Ignore everything else and run like hell for your exit. If possible, you can pick up your original civilian disguise on your way there before they start to look for a suspicious soldier. The exit is the small hole in the wall.
There aren't really any. At best, you can shoot your way through the muslim soldiers before picking up your sniper rifle and taking position. Depending if you want to collect the other gun that can only be gained here, you'll have to get back down and try to shoot the target from ground level once the UN soldiers enter the gates. (invisible walls prevent you from going to them beyond the gates.) And even then you probably can't: you're not allowed to shoot them and using aneasthetic on the guy on the jeep while dodging the others looking for you is most likely impossible.