as neospartan said, you need to go offensive from the start.

you have a substantial army that will very quickly put you into debt, while your enemies the aedui, are getting massive cash injections from the script so if you dont kill them quick, they will out-recruit you and its game over.

you can win most battles against them by simply lining up your infantry in front of the enemy, putting them on 'fire at will', and raining javelins on them. a few cavalry charges and they are routing.

after the aedui are destroyed, take as many poorly defended rebel cities as possible. you should be able to control central and northern gaul very quickly.

start training levy spearmen as soon as you come out of debt, and work on getting high level barracks in your highest population town so you can train heavy infantry.

in my campaign, the romans attacked me in 270bc, sieging massilia and meidolanium at the same time.

they are a difficult opponenet, as they have armies comprised largely of triarri and princeps, which you will at the time have nothing to compete against them with. your strong javelins which slaughterd your bare-chested neighbours are now useless against their armour.

the solution is to -never- fight them in seiges when you dont at least grossly outnumber them, especially in stone walled towns.

run an army comprised largely of generals, at least 4-5 of them including your faction leader and heir. as well as 10 or so units of infantry. basically you want a line of infantry to absorb the attacks of the enemy while you muster as much cavalry as possible to flank them.

when you besiege well garrisoned towns, just wait there until they attack you with a relief army so that you can get the town without having to assault it.

people often complain about triarii, but the thing is they have a quite a significant weakness you dont see, which is that they have a very high auto-calc rating, which means that the AI overestimates their worth, and will a result attack you even when you outnumber them, allowing you some easy victories.

try to let them attack you. form a line in a good position, let them march to you, attack your lines, then get round the back of them and charge them until they rout.

the same sort of tactics apply to most armies and most situations, but i think the most important point is to not go on the defensive, as the AI is able to pump out a lot more units than you can in a very short time.