It was Umber who cast it, and I was playing on Hard. Though it's not a game killer, it was definitely annoying. He kept casting it again and again until he apparently ran out of manna. He didn't take advantage of my dispersed units though...
I wonder if the mana cost increase for tornado will end up hurting the AI more than us players, by causing the AI to drain all their mana without their taking advantage of players armies being split up and scattered (as seems likely to be the usual case).
Players can better adjust to the new increased cost of tornado casting (and then taking advantage of it to kill scattered units), so the change may end up hurting the AI more.
Another reason why the 'increased tornado mana cocst' change is a step backwards (other reasons include it 'dumbing' down the game instead of 'smartening up' the AI by having the AI do a decent job of reforming scattered armies, and taking advantage of our scattered armies).