It's easier to pass with 2 dancers, Brave Lyn and another cavalry to buff her. But I tried this to test if it would work for my new sub-account. In my sub-account I don't have a second dancer.
Brave Lyn - Atk 52, Res 30. Hp 40. After hitting red mage, 4 HP left. Mine is a +atk Lyn with summoner support. SI: reposition, desperation 2. I used HP+3 seal because she's -HP. Unfortunately you need this much atk, or you can't kill Cecilia in the end.
Olivia - Spur Atk 1. not sure if it's useful since my Lyn was never positioned adjacent to her. but after I remove this seal, Lyn couldn't kill Cecilia, missed by 2 atk (22*2 in this video, 20*2 in another try).
3. Brave-Ike: reposition. not important. I used him only because my sub-account has him.
4. Ryoma: you can use any red sword. SI: Wings of Mercy 1. I think I used it in the last step to hit the ranger. So I guess if you have a cavalry that's fine too.