I've played quite a lot of Aedui campaigns, so I know this problem all too well. Here are some things that might help:

- you could conquer Segesta and Bononia, have them both filled with full stacks mainly consisting of slingers and (later in the game) some archers. Rome will besiege you almost every turn, but often they will only use small armies. You just have to sally out and destroy their armies without engaging in close combat. This worked fine for me, but it gets really annoying after a few years. After the first 20 battles or so I'm usually tempted to use the auto_win cheat.

- another possibility is to raid Italy very early on. If you defeat the main roman stacks, it's very easy since most cities are not guarded very well, most will have only one FM as garnison. In the last campaign I played, I took all roman cities except Taras and Rhegion, exterminated the population and destroyed all buildings and then let them rebel back or be taken back by the Romans. This was a serious blow to their economy and they left me alone after this for many years. It also provided me with a lot of money, so I could conquer and build up Gaul. You should however not destroy the gouvernment buildings, because if a city rebels back to the Romans, the AI will not rebuild the appropriate gouvernment and will thus have no access to their factional units in the later game.

- I have also experimented with a cheat to buy me some time: I used the create_unit cheat to fill up Bononia and Segesta with Gaesatae at the start of the game (I know it's totally unfair ;-) ). That way the Romans conquered the south first before they eventually expanded northwards. It will take them some decades before they manage to take those cities, so that you can expand rather peaceful for a while. It is however a cheating game, so if you want to play seriously, you shouldn't do that.