Why not take a galley or two full of spearmen to pillage Ashur's Iron, then fortify on the hill? They would throw a lot of units at them, especially if we do that first. Then, at the same time, attack Nineveh and their other Iron source. The AI will have no idea what hit it and will probably just wander back and forth while our Immortals and hopefully horsemen too, rape and pillage. We already have all the units we need to start other than the horses and galleys.
EDIT: Either that or sign ROP with them for 26 gold. It would be faster, and if we conquer the other two AI's quickly enough, we can still save our rep.