The battle isn't too difficult. After defeating him, the main path is in the direction of the red arrow. There's a chest at the green arrow location containing materials.
When you reach the target point on the main path, press V while facing the wall on the right to reveal hidden text.
Once the text appears, you can proceed directly, or collect materials from both sides of the path ahead.
After collecting the materials, interact with the text to move to another area. Keep moving forward until you reach the save point and save your progress. First, go into the room on the left to grab some materials.
The door on the right is locked, so ignore it for now. Head forward to open the treasure chest by the roadside.
Opening the chest grants you the second Oni weapon, "Oni's Strong Bow."
