In all these films, the main and supporting characters are like this. If the the main or supporting character is a girl, that the same rule applies. May I remind you that in all these movies, the female character is surrounded by men who are doing the exact same thing.
In "Vikings" the first scene involves the main characters taking on a bunch of soldiers at the same time, In 300 ROAE the main male character probably killed a whole lot more people than 50.
Just because women on average are physically weaker does not mean that every single woman on earth is weaker than every single male. History is also filled with elite female fighters with one example being Tomoe Gozen, a female Samurai.
Why else would they put in such historical inaccuracies? In these historical films no woman would be able to beat a fully trained soldier in combat let alone 50. For example in the series 'vikings',the she-elf in the hobbit, Atlanta in hercules, Artemisia in 300: rise of an empire etc.
It is an attempt to show that women can be better than men at fighting and that they are just as good a soldier as any man.