Not sure if it's best to try making a client state of that enemy in this situation, if it means you're obligated to come to its aid if it is attacked by as third party (which you probably cannot honor for many turns henceforth, because you need to shift your attention elsewhere). Once that walled city is in your hands, you have a good defence position in this direction and then I think you had better try making peace with the remaining portion of that enemy, so as not to stretch your realm too thin. Without mercenaries, getting the Boii to help in this endevour is probably critical. So, unless you're in position to push on towards the larger, walled city directly after taking the first settlement, you may have to disband the mercenaries until your forces are strong enough to move on that larger city. For economical reasons, these should not be recruited until you're ready to attack. In both these cases I believe one has to rely mostly on better-quality mercenary units to break the opposition in battle.

My current theory about how to manage the early campaign is to let Brennus stay behind protecting the home country or in reserve for reinforcements (of lesser quality) while using other generals to spearhead the first conquests, which I think should be directed against the first settlement to the west and the larger city further to the west. And it often becomes worse if you use one of the strongest units at your disposal, the one which includes Brennus, as he gets killed quite easily, leaving you without important historical character with which to relive the historical conquests for that faction. Tried perhaps half a dozen times without success, but mostly due to civil war erupting so easily in that challenging situation. Yeah, the Senones in ROTR is a tough nut to crack on H/H.

Rome 2 rise of the republic crack