VH/ VH on BI and I got the best challenge yet. You got 5-7 stacks each for every horde factions roaming the map, Roman cities are tempting but they can unleash their Comitatenses on your single fledgling city. The only faction you're sure not to attack you are the dead ones.
I have one game in five that survived to take more than 10 provinces since I started VH/VH, still getting used to the pace of BI.