After defeating the Chained Ogre and General Tenzen Yamauchi, find the next sculptor’s idol to the left of the broken bridge, down to the ledge in the valley. This will serve as your starting point. Here’s the rundown of the farming method but feel free to apply your own strategy or enemy priority.
From the Underbridge Valley sculptor’s idol, grapple to the next branch and kill the lizard hanging under the bridge.
Kill the first guard walking to the stairs. This NPC will be spawned by the game from the same starting position, walking towards the stairs every time you start from the sculptor’s idol.
Use the Grappling Hook to pull yourself to the branch near the first guard’s starting position. This will bring you to the ledge where the noisy lookout is located. Get rid of him.
From there, the swordsman with the strawhat will leave his post. This will give you a free stealth kill to the gunner looking inside the fortress. After killing him, get rid of the next guard by the stairs.
Next, get rid of the strawhat swordsman and the gunner beside him next.
Finally, go to the ledge behind the last enemy inside the fortress walls then perform an aerial deathblow. From here, you can go back to the sculptor’s idol you last communed and repeat the process.
Ashina Outskirts - Underbridge Valley Farming Location