Yojo Senki: Saga of Tanya the Evil

Tanya Degurechaff is a young soldier infamous for predatorial-like ruthlessness and an uncanny, tactical aptitude, earning her the nickname of the "Devil of the Rhine." Underneath her innocuous appearance, however, lies the soul of a man who challenged Being X, the self-proclaimed God, to a battle of wits—which resulted in him being reincarnated as a little girl into a world of magical warfare. (myanimelist.net)