Genova no Deshi: 10-byou shika Tatakaenai Majutsushi, Nochi no "Maou" wo Sodateru

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. Now, Valen can only use his former powers for 10 seconds at a time, and he's awkward around women. His apprentice, Ria, is a magical prodigy who forced her way into becoming Valen's apprentice, almost like a wife barging in. However, she's determined to hide the fact that she's a woman at all costs. When Ria reveals the shocking truth that "Valen's master is still alive," their strange journey begins. But for some reason, their adventures lead to the potential destruction of the human world!? This is a fantasy adventure of a once-invincible mage with a 10-second limit and a cross-dressing girl with a secret!

(Source: Shueisha, translated)