I would put Genghis Khan against Q. Fabius Maximus. While Q. Fabius Maximus' tactics might work against a deliberate opponent, scouting and delaying tactics would not hold be effective against the onslaught of the Horde. I can see it now:

Scout: Maximus, sir, we've scouted a large number of...uuuggghhhh *Arrow throw the chest, as Maximus looks up and sees 30,000 savage horsemen bearing down on his camp*

The legions would be destroyed in their tents.

Brilliant commander for the ends he pursued, effectively harnessed the technology and skill of his people to wield one of the most terrifying forces of all time.