First of, what do you mean by "entire" ICC. ICC is full of mindless ghouls controlled by the LK, they are basically trash. We cleared ICC many years ago.
Second. The Lich King is not powerful in 1 on 1 combat, Arthas Lich King was because it was a mix of several strong powers. Bolvar LK is merely keeping the scourge under check.
Sylvanas has acquired a power which we are about to learn about in the oncoming Xpack, why do you question something you simply didn´t learn about yet, people?