Ser Cauthrien is one of the hardest enemies in the game. She scales up to level 19 so it is not recommended to begin the quest Rescue the Queen until your party is at least level 19. If you are having trouble dealing with her, this page may offer some insight and tips on how to defeat her. If you are really unable to defeat Ser Cauthrien during Rescue the Queen, it may be a better idea to surrender and engage Ser Cauthrien again just before the Landsmeet.