Youjo Senki (Novel)

Synopsis

From Yen On:
High above the blood- and mud-soaked trenches, a young girl pits herself against army mages in high-stakes aerial duels with bullets, spells, and bayonets. Her name is Tanya Degurechaff and she is the Devil of the Rhine, one of the greatest soldiers the Empire has ever seen! But inside her mind lives a ruthless, calculating ex-salaryman who enjoyed a peaceful life in Japan until he woke up in a war-torn world. Reborn as a destitute orphaned girl with nothing to her name but memories of a previous life, Tanya will do whatever it takes to survive, even if she can find it only behind the barrel of a gun.

Official English:
Yen On, Tapas

Synopsis & details from MangaUpdates