What's not making sense to you? Scott is obligated to combat all 7 of the league of evil exes which control and determine the relationship fate of Ramona Flowers, if he wishes to date her. This includes fighting Roxie Richter, the bi-furious ninja.
Fed up with the league and Scott's self-pitying ***, she fights for herself, which unfortunately goes against the rules, so Scott ends up defeating her by touching her "weak point", defeating her, and making her orgasm, and subsequently burst into coins.
Any questions?