As was at the start of the fate route, Archer's wound from Saber doesn't heal properly maybe because he gets the majority of his healing due to Avalon or something. And Shirou did learn his basic fighting skills from saber. At the end of the day, Archer has a big disadvantage against Saber.
I was confused for Medusa as someone had said that she was on-par or probably more powerful than Iskander(I don't really agree though, and it was probably Ourai who said it). I think that Lancelot will also be a close match.