Its easier then I've first thought it would be. Playing as the Date you get a smith at your starting location and better no-dachi units. Your task should be to be able to recruit those as soon as possible, while on the other hand, the smith will be upgraded to maximum. This way the no-dachi get +3 armour, which causes them to have a total amount of 5 armour in the game.
One unit of my no-dachi can kill 4 opponent yari batallions, because of there banzai charge they can catch up archers pretty easy. Even cavalary are no match, after their charge which causes some damage, they do fall like flies. The only problem are katana-units, but the AI rarely recruits them and expecially in the early game you will only face 1 or 2 of them.
So Basically all you have to do is have about 10 no-dachi in your army, rest should consist of cavalary to take down archers and the general.
...and the pic above was with a wrong army composition -> to many archers, who run out of ammo after some time. Each No-Dachi killed about 600-700 units in that battle.