How and why does Wilhelm win against Theresia?
Judging by character synopsis on Wiki, it was not covered by all. He simply left during Demi-Humans War, and then returned and defeated Theresia.

One thing - I'm not sure, that Divine Blessing of Sword Saint gives its owner perfect swordsman skills. I suppose, Theresia only had certain fixed swordsman skill level, obtained by blessing. But Wilhelm spent years for training and a lots of effort to gain his skills. So, it is not that strange, that, if taking into account only swordsmanship skills, it was possible for ordinary human to overcome Sword Saint in duel. Maybe, if Theresia were to use any of other her blessings, she would defeat Wilhelm.