In the Kings Cross chapter, Dumbledore said he was the more skilled wizard and Grindewald knew it. He also said that Grindewald knew why Dumbledore wouldn't face him: because of his sister (which was said in the book), but NOW, it was because Dumbledore still had feelings for him.
So then the Elder Wand would win against anyone who was more skilled... hm.... So then what was the point of elder wand?
I think we can chalk that up to inconsistent writing from JKR.