High School Prodigies Have It Easy Even in Another World!, Vol. 4 (manga)

After spreading the word of their religion and garnering support from the people, the high school prodigies prepare to head into war with the Empire at the close of winter. However, Duke Gustav throws a wrench-or rather, a spear-into their plans when he suddenly launches 'Rage Soleil'! Barreling toward the Findolph domain, the magical weapon's furious flames threaten to burn the region to a crisp. Will Gustav have his way and exterminate those who stand against the Emperor? Not if the prodigies have anything to say about it!