So I did manage to conquer Russia, with just Bandits - on turn 41, in a First Game. I did not even need to do the event to avoid freezing to death in Russia. Bandits is really that powerful. Obviously need a huge chunk of luck with the turn order, but it is certainly not the hardest fight to win - I even managed to win that with an S-rank victory. All this without suffering even a single unit being wiped out on the battlefield.
After conquering Russia, and fetching yourself Gilgamesh and Enkidu 'optimal' becomes blurry to define, since you also need to do events to get all the items for the next playthrough, as well as recruiting the Ancient Heroes, and the more obscure ones, such as Archimedes and Cambyses. Once you have done Marco Polo's fourth event, money starts to roll in. Plan your turns so that Marco fights in every turn at least. I suspect that a First Game can actually be won inside 50 turns - as the final battles are not that tough.
Technically, you can farm infinite money from the Jiang Ziya recruitment event. Just bring along Marco Polo, Himiko, and a bunch of arts characters (Sun Tzu especially, as it is even easier when you have unlocked her passive; also Encourage is good for your bank account). Himiko for the healing boost - you may need it, Nostradamus for the critical boost. Also Magoroku is a decent shout, since the doubling of brave generated can be rather useful as well. Occasionally you do need to feed Jiang Ziya some brave, to avoid Marco Polo being attacked. If Jiang Ziya has 5 brave, she'll summon one of those orbs. Which can then be farmed for money by Marco. This in turn can be used to raise a lot of characters to respectable unit counts - I think in the Japanese version, I managed to farm 16M from that fight alone. That would be enough money to raise every character to 5000 troops.
At the moment busy working through the events to recruit the last Ancient Heroes (Atlas, Cu Chulainn and Jiang Ziya; yes, I am not pressed for money at all), and happily abusing Marco Polo's money making prowess. I am also working on collecting all the items, particularly the ones that are rather useful even in a first playthrough (Gullinbursti, the Jewel Box, both to farm money as well as to make some boss fights easier and anything to drive up damage from Marco Polo, such as the Crit damage +50% for Marco Polo items, but also some passives, and support skills that increase her Attack score).
By now, the game is 'broken' - careful playing of most battles result in S-rank victories within 5 turns, and since one can know how many battles are the result of events (character or otherwise), and what enemies one has to fight in them, the only unknown is whatever Pandora throws at you. But those Pandora fights are laughably easy (it has gotten to the point that I use Oda Nobunaga to get the Army of One & Bombardier bonuses - free units, so each of those fights is worth approximately 50k already!); in effect if you intend to maximise unit counts, it is a Marco Polo management game.