If something is not included, assume you don't have to do it.
1. Kill the 3 nuppepou
2. Kill this room

3. Trigger the trap

4.Open the 2nd chest

5. Open the key chest

6. Open the door with the key
7. Kill the Ungaikyou


8. And finally, Kill ahriman

Enjoy :P
Note: For step 4, you can shoot the chest from the main path with a bow or wand.