There is one question that needs to be answered here...
Does the "Six Samurai" monster survive the attack?
If yes, then this is what happens:
When "Ryu Kokki's" effect attempts to destroy that "Six Samurai" at the end of the Damage Step, the controller of that "Six Samurai" can use it's effect and destroy the "Grandmaster" instead. That "Six Samurai" will live.
If no, then this is what happens:
When the monster is destroyed in battle with "Ryu Kokki", then the controller of that "Six Samurai" needs to make the choice of either using the effect to save it or not. If they use the effect to save it, then the "Grandmaster" is destroyed instead. THEN at the end of the Damage Step, "Ryu Kokki's" effect kicks in and destroys that "Six Samurai" anyway...lol.
If they don't use the effect to save it, then it can't be destroyed again by the effect of "Ryu Kokki" since it's already been destroyed by battle...lol.
Fun times were had by all!