I played it as the Francs, I first settle as much land as i can, then i went all nice on the Roman to destroy the german, I don't like the way Romans are to the south and german to the north, I felt a trap there. Once I eradicate the german's 8th city I made peace with everyone, building an army big enough to deal with the roman legions, then I attacked.... The counter attack was a big, 1 and lonely heavy cavalry rushing on a poor worker i sacrificed. For some reason i forgot the warning given at the start of the scenario and was too shy to attack: Rome is cripple with corruption, they must have at most 2-3 shields a turn in their far-off cities. The hardest part was to secure the victory location from the other barbars. I hate to see friendly stack of death roaming my land. Be ready to be a traitor and rape your RoP, aim at 8 cities in at most 2 turns, then you won't have to worry about a counter attack. Don't accept MP, the other AIs will hate you if you destroy a civ before the 20 turns...And seeing there is nearly no culture in this scenario, most of the time you won't even have to rape your RoP, just use cavalry.
Worked for me on regent.