[theiconoflaziness]

Well your main problem is your not following orders, one of your PRIMARY objectives are to avoid being seen to maintan stealth.

IT's actually quite easy, at the very start of the mission before you enter the courtyard, there is a toll booth on the far right side that puts you on the right side of the map on the other side of a wall. All you have to do is move slowly staying beneath the wall until you get closest to the front door and silently snipe the MErc's closest to the door, you won't evenhave to worry about the merc's in the middle.