I think it's
Baalzebup who gave me this grinding tip, but basically just ready Divination talismans at all times, and use them when exploring/playing normally. (Recast as needed, refill when you reach a shrine and recast, etc.).
Onmyo proficiency is kind of whack, it increases when you do damage while the spell is active. So if you cast it, damage and kill a bunch of enemies, you gain proficiency points. Eventually you'll unlock the mission.
Fair warning, there's a follow-up mission later that's like, 250k points...