It is said Gul'dan was a mortal Warlock who's power stretched beyond the extent of any mortal Necromancer/Warlock in existence, but he did not even attempt to battle or kill Doomhammer, a completely normal warrior, in the numerous times Doomhammer cornered him. Eventually, he was slain almost effortlessly by lower class demons suspected to be Naga.
I just don't understand.