BLACK HILL, outskirts of Baghdad, April 8 — It took an anti-tank missile to blow a hole through the steel-reinforced wall surrounding the Special Republican Guard's headquarters at the foot of this hill overlooking the capital. An Iraqi soldier on a roof — or perhaps two or more, in the chaos it was not clear — opened fired as Company B of the 101st Airborne Division's 3rd Battalion, 187th Infantry Regiment poured through the breach this morning. Spec. Sylvester A. Prince, 19, was among those who rushed the sprawling complex under a volley of covering fire aimed at the roof....