Mordremoth’s weakness was his mind. Kralkatorrik is his own crystalline blood, but what was Zhaitan’s weakness?
Cannons. (Hey, I used the correct form of the word this time!)
I'm kidding, of course. I believe it was cutting him off from his food supply (mouths of Zhaitan) and basically blinding him (eyes of Zhaitan.) Then in his weakened state: Cannons. I think a lot of people forget the campaign in Orr up to the final fight, but IIRC, he's already been "weakened" by the time you fly up to him and shoot him off a building.