Well, just because he is good with both the sword and magic doesn’t mean he is good at using them at the same time. For all that confidence, it seems he really was just banking on an assumption that because she had limited magic she would have no means to fight back against a magic user. Given their connection to the prince, these knights were well trained with decent gear but never actually saw much in terms of real combat. They seemed more like glorified thugs than actual battle-hardened knights.