I pretty much do like Didz said above. I let them die trying to take my cities. Whenever possible I try to kill their leader on the first sally out with my siege and archers. That way on their next turn the Mongol army isnt as well lead and sometimes will just give up and go away. If they do assault they do it with less vigor and intellegence it seems. Take out the Generals and a Mongol stack is alot easier to deal with.