So it does seem like Akou is indeed Riboku's intended target.
If so, I'm not sure I agree with Riboku's reasoning of Akou being the most troublesome enemy in the battle. Now, Akou and his army are very strong. But despite what Akou is capable of doing, his tactical prowess, his way of fighting and his might - Riboku at Shukai almost sent him to the grave, he understood him completely.
Yet, there are other two individuals present here who back then at Shukai proved to be key factors in the Qin side achieving victory, I'm of course referring to Ouhon and Shin. Ouhon is a very polished commander who was able to overcome Zhao's pawns, Gyou’Un amongst them. Shin did the same and in the most recent war against Kanki, he made the most brilliant instictual move in his career, so much that Riboku got reminded of Duke Hyou. For the type of commander he is, Shin is probably the most troublesome factor for Riboku.
Both Ouhon and Shin have kept on growing, both are key generals for Qin now, both have proven to be extremely disrupting against the opponent in the right context. Akou is a chad but right now I'd be more concerned about these two.