The Roman army was quite a formidable fighting force. But that's not to say they weren't impossible to defeat. A group that had somewhat regular success against them were the various tribes in Germania. Warlord Games' supplement to Hail Caeser lets you bring those vicious tribesmen to your tabletop. And that book is now available in digital format.
The book focuses on the Varian Disaster (as the Romans called it), when tribes being led by Arminius ambushed and destroyed three legions of troops along with their support. There's rules for the various troops that participated in the battle, as well as scenarios and special rules for recreating the battle, itself, on your tabletop.