Yeah like everyone said, you have to face a inexperience rome player to win with a weaker faction. Against a experience rome player, he is usually aggressive with his archers and he shoot the right thing, knows where and when to commit his inf and when to stop chasing skrimishing light horses.

I have to admitt that those time when I beat rome, my opponents are usually over confidence with their units or simply underestimate the army they face. Against a experience player, I don't stand much chance.