After copying and assimilating 50% of Khinux, now Uno's swordplay patterns, Karsha finally manages to break past his current mastery and become a Grandmaster Swordsman. He is now at the critical point in every Swordsman's journey. The Grandmaster stage is where they create their battle style. It's the point in their journey where they create the style they will be using during battles.
Swordsmen regardless of how they were brought up, no two people can have the same 100% battle style. Even with Karsha's imitation skills, he wouldn't be able to copy and use the complete style of swordplay from any opponent. The best he can do is copy and use it to enhance his own swordplay.