EMIYA likely fails miserably, unless the Exclipoor has power close to that of the original. And even then, Excalipoor has a drawback in that it can't recreate and crystallize hopes and dreams of mankind like the real Excalibur, which are proven to work incredibly well against Walpurgisnacht.
No, you need Saber to take care of that one. Or maybe Gilgamesh with Ea.