1. the warrior looked tough, but was probably quite a bit weaker than her, sword saints aren't really fodder
2. the demon king let rudeus use his strongest attack to test rudeus, demon king would have won easily if it was an actual fight.
nina does clearly does have an overinflated ego this chapter and seeing rudeus in action will rightly humble her,
but she probably would have given Rudeus some trouble if she fought him in close range (rudeus would still win since he has foresight).