Fantasy Maiden Wars Kou - Flandre Secret Event

The event you get on Marisa’s stage 13 after you fill the Gensokyo Chronicles. It’s possible to fight Flan I think, it just requires you to have a near god-like Marisa dodging those one-hit Laevateinns and overpowering Flan’s HP Regen since even Magic Missiles do about 1000 damage.