As everyone else here suggested, blitz the Arverni. I combined my Leuce Epos into one army under your faction leader along with the Bataroas. Disband several of the Lugoae, they're mostly useless in battle against the Arverni. Attack Viennos first, you will probably be attacked by the Arverni. Defeat them in the field, take the town and move south towards Gergovia. Take that quickly as well and then again move quickly to the final Arverni town. You will probably have lost a good deal of your starting army through these battles but also have doubled your total of starting provinces. Now move to take Dariotitum on the Atlantic coast. Aventicos is a tempting target but its garrison is larger and there is a large rebel army wandering nearby. You might lose too much against these forces so take the lighter garrisoned Dariotitum which also gives you another port and one that will eventually produce your fleets. Now you will be turning a decent profit and have a decent sized army in the north and another smaller one in the south.

This is the troublesome point of the campaign. Likely you do or will soon border the Sweboz. Their raids across the Rhine are annoying so you will need to keep a good force near to the border. If you have enough troops, move to take Aventicos and then on to Lemonum. Avaricum is nice but has stone walls and a large garrison. Lemonum has a port, a smaller garrison, and can produce the wonderful Pictone Swordsmen. Get mines built in Aventicos asap and prepare for the Sweboz. If you have extra time target Avaricum and then the Belgae towns.

Meanwhile in the South you have inferior troops and a superior enemy in the Romani. Try to take Venetia as soon as you can because the Romans will be coming and you will likely be retraining your units here so much that you will need two sources of troops. Wait for the Romans to take Bononia and then counterattack, their army will be weak and you will likely kill a family member or two. What you want to do is take Arretium and Arimnium and destroy their barracks. Recruit Botroas, your spearmen are not sufficient to take on Hastati or Principes, let alone Triarii or Pedites Extraordinarii. This is a tough point in you campaign because funds will be low and you will have to keep the superior northern army north of the Alps conquering rebel settlements and fighting off the Sweboz but if you can beat off the Roman attacks, especially the annoying mass merc recruited armies and sack Rome, your campaign is all downhill from there.