As far as power goes, the Burning Crusade is probably something like: Sargeras > Kil'jaeden / Archimonde > Gul'dan > Lich King > The Rest. Meanwhile the Black Empire is more like: N'zoth > Xal'atath > Azshara > Yogg > C'thun > The Rest. So in that setup, Azshara is essentially on par with Gul'dan. She would defeat the Lich King.