Youjo Senki: Valhalla

It is 1914, and Edward has been transmigrated into a German Infantry Soldier named Diedrich Griswald. Waking up in the trenches of the Western Front, he tries to stay alive in this hellish war. But what is that flying figure!?

Looking at the mages who are flying around, blasting explosive rounds on the enemy position. Diedrich finds himself fighting in a slightly different World War I.

(Forget to copy the source, so let me know if any of you can find it.)