Bronn is nothing short of a killer. The men you bet on in these bouts are always, always those that do not mess about. Belwas is great, but he would not last against Bronn. The whole point of Bronn is that he is everything a knight is not, and prepared to think on another level when it comes to fighting - he would figure out Belwas weakness and exploit it, just as I believe he would have taken down Gregor Clegane (and there are not exactly many people in the whole series you would give a chance to against Gregor, one on one! I wouldn't give a fig for Briennes chances of beating him, or Balon, or Arys)