All I understood is you bragging about how "supposedly" your strong, and you want to get this girl.
First of all, it's both the girls fault and yours. She's the one that stays with him and thinks they have a connection, even tho you claim that he yells at her/beat her.
Also, how do you know how much he weights? Did he tell you personally or are you just ASSUMING he's 265pds. And, I don't see how the fuck beating him up will want to make her become your girlfriend? She'll assume that you might be someone violent and will be less attracted to you.
So my word of advice: Just let her be. You can't really force anybody to like you/quit someone. Just give it time, she should get tired of it and leave him.