In Chapter 16, you can get the Dark Dragon Blade (but cannot use it). Once obtained, that is about the 1/4 chapter marker. When you resurface with Rachel's help, you will see an intermission sequence that reveals who the Dark Disciple is. He is not very difficult, especially when compared to Doku. If you die, you do not have to re-do the entire chapter. When you get to the Dark Dragon Blade, it will start to show an intermission sequence. Repeatedly press Start, and it should skip to the scene where Rachel is helping Ryu out and you can just start fighting the Dark Disciple again. Note: This was done under the Ninja Dog difficulty setting.