Aah that level. It's hard to know exactly who you are talking about. My friend and i redid that level yesterday and i think i know who you are talking about. After sniping all those guy's while hiding beyond your door. Go down and get the armor. After that jump out of the window and to the extreme right there should be a pile of boxes and a sniper standing right there. Just look around till you find him. If it isnt him well then keep looking.