Best way to make money is the arena fights. You start only making 100 francs a fight, but after about 20-30 fights every fight you do gives you 2000 francs. Make a little money then go companion hunting. You need a group of companions that don't hate each other, 3 is necessary any more is a plus but you absolutely need 3 companions and you want them to be low level. This is so you can mold them into a doctor, tactician, and scout respectively. Every time a companion levels increase their intelligence, even if they are not one of your main 3, this is so you can put more points into trainer, which makes it so you can level units just by waiting a day. Once you have your companions, equip them at the french military depot with cuirasses, cuirassier helmets, cavalry gloves, and a light officer sabre, buy horses from the market in town because right now those horses are better than the depot ones, go to England and in the military depot buy British infantry rifles(for some reason my companions equipped with carbines and swords on horswes could only use carbines, with rifles not an issue), also 95th rifleman pants (some reason French max pants armour is like +16, the 95th pants are +20 or +25). Once everyone's decked out, start going from place to place doing arena fights. Use the fast travel. Kill any bandits and level yourself and your companions. Once you've done enough grinding, I like to get myself to around 22 and my 3 main companions to have 6's or 7's in their important skills.
Just for clarity the increase in arena money is as follows, minus the info as to how many guys you have to beat to get to each level, as that I am unaware of.
1st level=100 francs
2nd= 350 francs
3rd= 750 francs
4th= 1000 francs
Final= 2000 francs