Excalibur isn't as much "overkill" as you're making out. And, remember, Rider has Shirou to assist her. Who, as you might recall, was able to use Rho Aius to allow her to defeat Dark Saber in HF.
So, yeah, she can totally win that battle. Not that it matters because, again, no killing without permission, and I'm not intending to let Rider get killed.