Saijaku Muhai no Bahamut

A former prince of the fallen Arcadia empire, Lux, stumbles upon the bathing area of a female dormitory and witnesses the new princess, Lisesharte, in a state of undress, prompting her to demand a Drag-Ride duel. Drag-Rides are ancient mechanical armors recovered from ruins worldwide. Once known as the strongest Drag-Knight, Lux is now labeled the "undefeated weakest" due to his refusal to strike in battle. Following the duel, he is required to enroll at a female-only academy that educates royal students in becoming Drag-Knights.