This actually works!! OMG, I prepared for big battles like this:

- lock your borders with forts, build up some military to compensate the graph of others. Small early wars for good provinces are good, but keep your total below 12-15 for a challenge.
- keep peace, not alliances, if romans declare war offer alliance to one of them (the others will offer cease-fire) and break alliance
- build up your cities to level 5, all of them, using the peasants train & disband to get population around (easy on setting: Huge)

Combining with Darius' advice:

- Give insane amounts of cash to other empires as your cities level out. Especially those you'd like to conquer.
- Build up your military, and BOOM, Total War!!