He was still a decent sorcerer, given that he maintained the portal and that Illidan feared Mannoroth would catch him if he sensed the teleportation.
I'm not sure the Lich King is absurdly strong if he's weaker than Lei Shen. As for weapon comparison... I don't know? A charged Doomhammer (also considered an artifact in the Legion) barely scratched Mannoroth's wing, so we don't know how well Frostmourne will perform.
As for magic, I'm not sure. The Lich King seems to be a master of ice and death, but I'm not sure if this will help against felfire and a huge demon.