Undefeated Bahamut Chronicle - S01

Lux, once the prince of the fallen Arcadia Empire, accidentally stumbles into the bathing area of an all-girls dormitory and sees the kingdom's new princess, Lisesharte, undressed. Enraged, she challenges him to a duel using Drag-Rides - ancient mechanical weapons dug up from ruins across the world. Lux was once known as the strongest Drag-Knight, but now he's called the "undefeated weakest" because he refuses to fight back in battle. After their duel, he somehow ends up enrolled in the princess's academy, a school exclusively for royal women training to become Drag-Knights.