This is an example of how to S - Rank on NORMAL Difficulty using POST GAME characters and items. I can do it in 2 turns, but it really is not necessary as doing it that fast is luck based.

Name: 学び舎のヴァルキュリア
Map Area: ランシール士官学校
Difficulty: ★★★★★
BOSS: Valkyria - Alysse （ヴァルキュリア ： エイリアス）
Ace : シバ鬼教官
Ace Drop: テイマーSPG (Tank Part ?)
S - Rank: 3 Turns