I recommend that cavalry guide. I've been trying it with a few units of Illyrian light horse against small rebel stacks in my campaign, and the Illyrians tend to rout many enemy battlelines in a few seconds. The enemies do not route instantly, but by the time the last units hit home in the fourth charge in 10 seconds... yes, even from the front levy morale tends to break.

Wouldn't work against a proper army with a real general and is unrealistic for Romans anyway, but the piledriver works great for swatting small groups of enemies.