AI is never challenging

[Geticus]

Definitely boost the AI to Vh campaign and Hard battle, that is my personal favorite, Very hard battle just warps things out of all proportions IMO.

The Romans are one of the best factions in Europe, if you want more challenge I'd recommend, as others suggested, one of the Gallic factions, or maybe KH.

If you insist on playing the Romans, I'd recommend that you fill your armies with a more historical roster, i.e. armies during the 3rd century were mainly Romano-Latin, consisting of legionary infantry with 2 rorarii/velites, 2 hastati, 2 principes, 1 triarii ratios with little or no pedites extraordinarii. I believe it is historically very dubious that Italy produced any regular infantry outside of Rome that outclassed the best Roman infantry, so I basically reject the pedites extraordinarii concept especially during the 3rd century, later on it becomes more plausible. Also your main cavalry should be Roman equites, and not Equites Extraordinarii, Campanians or Tarentines given the mediocre loyalty of these city-states to the Roman cause during this period. The fact is that Rome relied primarily on Romano-Latin forces during the 3rd century and the Polybian period, and heavy recruitment of the speciously overpowered extraordinarii is to me both nonhistorical and too easy.