Once, long ago, it was decided in Ransei that there was to be a test carried out to determine the junior Warriors' of each kingdom's strength. This test would have a Warrior from each kingdom get a random castle and demonstrate their skills on the battlefield. The Warlords were not allowed to help.
The time for the test has come again. Shingen told the Warlord gathering that Yukimura is the best there is, that he should take this test.
And guess what? This is basically another version of Hideyoshi's storyline. Man, Nintendo's loving you to do this. Just this one, the main play, and Hideyoshi's line can take about sixty hours total!
And, as ironic as it seems, we can't help you here. As the test dictates, you will receive a kingdom at random. You'll have to conquer all seventeen of the kingdoms. All you can do is learn Pokémon weaknesses and strategy, learn to search for good recruits, and so on. Good luck!