東方玄夢妖譚 ~ The Nightmare of Rebellion - 小鹿倉紅葉 見参!
This would have been easy if she doesn't have a fricking HIGH Evasion! On the defensive side, she has a low Magic Defense so Magic is the way in this fight.
Don't bother using Control Land unless you feel like doing it and instead cast Accurate Weapon so landing hits will be slightly easier, Active Mental to amp up in the next turn (or add Keine's Three Divine Treasures skill too) and Kandachi's Lightning Wall to reduce the damage whenever Momiji uses her Lightning attacks. One spell you need to be on a lookout is Mega Stroke, which deals 600 (or 700 if she casts Control Land Lightning!) and that damage alone can kill one of your party members!
She is weak to Water, Fire and Blunt. Go for Chasers as always but this time, set some to Main Doppel Magic (Flandre, Suwako, Kogasa and Patchouli is a must) while others go for powerful Magic like Detonate and Cold View that does decent damage during Chasers.
If she used her Last Word, you're totally screwed as hitting her will become harder even if you used Accurate Weapon and pray to the screen you'll land some hits on her. If she's too fast for you, use Hatate's Tengu Field to ensure the party will act first.
I didn't plan on going for a dual elemental Chasers but I wanted to sure she goes down without ever casting Control Land nor her LW.
Now that third seal has been unlocked, the gate is opened and we can get through the next area! Great job, girls!