After completing the last mission, fast travel back to the Survivor Camp you met Yuna at and head northeast to the Old Yarikawa Ruins; she will be on the side of the road, arguing with Taka. Speak with Yuna to begin the mission, which will have you looking for the archers that went out on their own from Yarikawa. As soon as you gain control, move straight ahead to find some dead Mongols. Examine the one body to see they died of arrow wounds, then look nearby for tracks. Follow the tracks up a wooden walkway and over a small ridge, where they will lead you to a house.
If you use your Focused Hearing before Taka arrives at the front door, you should see a surprise waiting for you on the other side. A bear will break through the door and begin attacking you, so quickly equip your bow (or if you want to be brave, use your katana) and bring it down. When the coast is clear, enter the house and examine the few points in there, then exit and speak with Yuna. It seems the archers are close, so search around the campfire to see if you can find any clues. Nearby is a cart with some explosive barrels, then you will be able to find more footprints.
Follow those a little bit and you will see two archers slowly moving ahead. Keep up with them to arrive at a ridge, where they are watching some Mongols. Survey the Mongol camp, then Jin will come up with a plan for an ambush. The Mongols will be coming up the road, but you should wait until they are a little past the one cart with explosives before firing at it. Despite them leading the archers they took captive, the explosions don’t seem to faze them, so you don’t have to worry about them. If timed right, the explosion should take out a good number of Mongols.
Drop down and pick off the rest of them, then get ready for some more waves of them. Since you have the Yarikawa archers on your side, they will likely snipe a good number of Mongols themselves, but that doesn’t mean you can’t join the good fight yourself. With everyone on your side, it shouldn’t take long to defeat all of the Mongols. After a few scenes following the action, the mission will end.