Originally Posted by
The5thVegetable
The power of the Scourge lies in their ability to turn their enemies' numbers against them. While the Lich King is certainly extremely powerful, his power lies primarily in his armies. Azshara eclipses Mannoroth in power, and the only members of the Legion that are beyond her in power are Archimonde and Sargeras- or rather, that was her power level 10,000 years ago.
Not to mention, we need to put how they fight into account! Azshara is a powerful mage, whereas Arthas' magic is far more limited in that regard and his persona power lies more in melee combat. In melee combat, Arthas could certainly prove dangerous, but with sufficient space between them, Arthas would not be able to lay a finger on Azshara unless able to close the distance quickly, and I don't see that happening unless, perhaps, he's mounted.