Genova no Deshi ~10 Byo Shika Tatakaenai Majutsushi, Nochi no Mao o Sodateru~

Can a former strongest mage, with a 10-second combat limit, and his cross-dressing female apprentice (desperate to hide her gender) save the world? The former strongest mage's apprentice is actually the Demon Lord!? This is the story of a certain master and apprentice on their journey. The master, Valen, was once known as the "Strongest Mage," but after a tragic event, he lost his beloved master, his sight, and all of his magical power.