You learn that you can't possibly defeat the true enemy on your own, so you search for allies in Hoshido.
Before the Battle
Before you start this battle, you might want to choose Paralogue 1 from the Next Battle menu. This will allow you to recruit a new unit and also level up your existing units. This can be helpful since you have such a small army to start with.
You should also be sure to buy equipment for your units, since they might have improved their weapon proficiencies by now.
Defeat the Boss
In Fort Jinya, you will have to defeat Yukimura to finish this battle. At the start of the battle, there are Sky Knights and Spear Fighters to the west and east. You can enter the fort now if you wish, but be prepared to deal with the Spear Fighters and Sky Knights that follow you into the fort.
On turn 7, a Ninja will appear at the entrance of the fort. This Ninja will want to open up the chests in the map then run away with the contents. To prevent this, you will need to open the chests yourself, or kill the Ninja. There are enemies near the chests that carry Chest Keys, so you will have to kill those enemies before you can open the chests. Once you open the chests, the Ninja will run away.
After dealing with the Oni Savage and Samurai in the first room of the fort, go northeast and you find a room with a Dragon Vein. Activate it to turn some of the tiles into healtiles. Then kill the enemies in this area to get a Chest Key and open the chest to the left.
Go back to the first room of the fort, then go northwest. Fight your way to the northeast part of the fort, where you can kill an enemy near a chest to open it up.
Finally, you can go northwest to fight Saizo and Orochi. Orochi is weak to spears, so Gunter, Azura, and Mozu (if you have her) are the units to send against her. Saizo is weak to axes/clubs. He has the Pyrotechnics skill, so if he attacks when at half HP or lower, he will lose 20% of his HP but so will your units nearby.
Finally, you will need to defeat Yukimura. He uses both a dagger and a yumi, so depending on what he has equipped when you attack, he will either be weak to axes/clubs or swords/tomes.