The later you leave the Romans the harder it gets for you. The britons are a faction who can produce very good quality infantry right away with Woad Warriors within a few turns (only problem is the 2-year training time).

Take out the Gauls ASAP and then Pick a fight with the Germans once you have all that Gallic territory. Work your way towards the Alps and take on Rome ASAP, hopefully you can catch them Pre-Marius and wipe them out.

When Italy is yours, pour East.