How do I know what unit can cast Tornado?   And what if that unit is in his city?   I'm still on the world map, I can only cast unit spells on the tactical map, so I have to get that unit on the tactical map before I can disable it.   

In other words, he can cast this spell on the world map but I can only counter it on the strategy map which I can only get to by attacking the unit but I can't get to the unit because he casts Tornado?  

And what's an Ophidian? 

Thanks for the info, I'm just finishing up my second game and am really enjoying this.   But the spell stuff... it's a little tricky sometimes. 

I tried the multiple stacks and it blew them all away.  

Yes, multiple stacks work.  Send each stack to attack the city from a different tile.  Also, have each attack a different tile of the city.  Also, steal a page from the masters of battle tactics: use feints.  A feint is a diversionary attack.  Begin your attack on the city by sending a sacrificial unit to attack.  Hopefully, this will draw out an air mage to cast tornado.  Rinse and repeat until no tornadoes appear - then attack with main army.  

BTW - I do not feel this is an exploit because it counters the handicap that your garrison displays when defending your city.  The garrison only shows up to defend against the enemies first attack of the turn.  Successive attacks on your city that turn will find the garrison absent.