Imo I think Semiramis's NP should be changed. Like, it's obvious that her NP would only work in Apocrypha and that's practically it. Avicebron is more or less in the same boat, but at least he could potentially (would be difficult though) manage to build it himself with time, but Semiramis cannot.

So anyways. With Hanging Gardens of Babylon she would obviously win without a doubt.

Without it, as it would usually happen, Kojirou would probably win. I'm taking the current power levels version btw, because the F/SN version is... quite weak to say the least. (He's absolutely impressive in skill, but he just lacks Servant stats. But in things like F/GO he's more "normal" now so that's the version I'm using.) It's not the best matchup for Kojirou due to Semiramis' magecraft and long range attacks, but I still think Kojirou would eventually be victorious.

Well if Semiramis have the Gardens I am sure she would win easily, Kojiro does not have anything close to harm her while she is at the gardens

But making the Garden takes up a lots of time and materials, something she probably wouldn't have in a normal war.

Kojiro is a beast in close combat and he will probably will against her way more easily

But if she knows who he is she can probably win by keeping her distance and using poison and magecraft, and considering Kojiro assassin is his weakest form, I would go with Semiramis

Gardens = Semiramis

Close Range = Kojiro