A heroic spirit will ALWAYS be WAY stronger than almost anything that exist in whatever the era they are invoked if their master has a good mana pool, this is even more true when taking in account heroic spirits that are well known, that have a great story behind them (exaggerated or not), are worshiped, etc. Nobu is well known (actually, only in Japan, for manganime fanatics, or people interested in japanish or world history, but you know), so if her master is a powerfull mage then it should be expected, but in all honesty any non-divine servant with a good master should be capable of defeting her, given that most of his ability are oriented to kill divine (of any level or kind) servants.