Fate/stay night

[Munsu]

well "famous" its said that he probably didnt exist, a similar person probably did, but not a sasaki kojirou, he is not even mentioned in musashi's book

That seemed to me like you were talking about the person not the fight like you now are claiming...

But whatever, there's a lot of speculation to what really happened in the supposed fight with Musashi, no one can really know what really went on, the link I had provided for the wikipedea explains some of the possible scenarios to what really happened... There are all sorts of different accounts.

In the ep. Berserker appeared, Rin said that he was the greatest hero in greek mythology.

and the only person I know of like that is Hercales, Hercules in roman mythology.

No, the master of Berserker herself said who he was, with name and all.