I routed a unit of Gaesatae last night, actually, without too much trouble. I just treated them like the juggernauts they were and swarmed then with crap infantry, then had a light cavalry charge thrown in at their backside. This, coupled with the recent death of the Gaesatae's general and a few routing units near to their melee, was enough for them to turn tail and face slaughter from the cavalry who had already engaged them.

Really, guys, it's not that hard. Just use your head.