Kimi to Boku no Saigo no Senjou, arui wa Sekai ga Hajimaru Seisen

From Yen Press:
The Nebulis Sovereignty and the Empire are embroiled in a war without end. The Ice Calamity Witch, a Nebulis mage of incredible power, and Iska, the youngest swordsman of the Empire ever to become a Saint Disciple, find themselves face-to-face on the battlefield. Each seeks to bring a close to this ceaseless conflict. Little do they know, however, that their meeting will be the catalyst that shapes the fate of the world.