Aside from his other achievements, I had been thinking (like others) that he was gonna have Shin outright slay Renpa but, since that wouldn't necessarily mean he surpassed Riboku, then Shin besting Kouen, a Riboku lv or Riboku lv+ commander, would do it.
Regardless of what he does, man, Shin has an absolutely freaking, stupidly insane task in front of him.