This boss is relatively easy but packs a pretty strong blow for its attacks and it has extremely high defense. Do not bother trying to bring the stats down. It will counter with a skill that negates all negative effects.
You may want to equip some light resistance accessories/armors. The boss uses holy breath attacks.
Have Byakuren enchant light or dark element on your melee characters.
Use all available buffs from Sanae and Byakuren.
Recommended party: Byakuren, Sanae, Youmu, Sakuya, (reimu or mokou)