And don't forget Shirou is just a 16 years old kid with 0 experience. It's just right he only has 1/1M chances of beating him if Gil was serious.
So we shouldn't take what Nasu said for Archer too even if obviously Gil would have won if he went seriously. I just want to say in that case Archer would have been harder to deal with, just like any others servants in fact, yeah UBW can't hit as hard as GoB, yeah his skills are just the peak of normal human beings and not heroes but please just don't give the feelings you're underestimating him too much and look at him like a trash who can't even win without plot-shield...