@K.:
The setup for Riboku's ambush was pretty weak. Mostly because it was not shown how they got into that position, but they left it up in the air, merely spoken words. However, not that I think Ousen would be ready, but I still feel he would have spectated the entire ordeal. Those words from Riboku, that Ousen does not know which tactics he employs, I think will play a greater role down the line. Makou now seems like a sacrifice, however it all falls back to Makou not being prepared, not having his glaive, sending the elite troops, etc.
Shin will go and stop Riboku, but to what end. At this point, the Makou army is getting annihilated and chasing down Riboku will just allow that to go on. I am aware that the narrator said there was no way to reverse the tide again. I guess it will be up to Mouten to regroup whatever is left of the Makou army and keep fighting. Although, Mouten vs Batei is more probable now.
Well, the opposite is also true. Riboku doesn't know every tactic Ousen can employ.
What if there's another detachment coming from Qin's center that will try to cut off Riboku?
Hard for me to believe that Ousen will be completely oblivious to what has happened for long and that he won't try to turn it into an advantage.
–- Update From New Post Merge ---
@K.:
Reading the chapter again, the first page when the soldier asks: "Are we under attack?" I think that if we would have seen more of the positioning and how Riboku approached this line would have had more impact. It brings to light that no one realized Riboku's attack even though they were in between Makou's soldiers. To contrast with the Hi Shin Unit, when they arrived Shin immediately raised their banners, to make the enemy understand that they were there. However, Riboku did not. I think these two parallels are important, because Makou and his closer retainers, knowing Ousen's way of fighting, would not be surprised that a detachment unit (like Shin's) would appear out of nowhere between their flanks.
Also by analyzing the pages were we get a zoom out view. Makou had maybe 5-10 thousand men behind him. AH! I just saw why!
! http://www.mangatail.com/sites/default/files/manga/6/218036//20170908014156220.png
When Makou sends in the remaining elite troops. He leaves behind him the amount of men mentioned before. But look at the position Makou is in before the elite troops start going forward. The elite troops were basically the left and right wall preventing Riboku from going in. So again, Makou…talked too much.
Now if you could only explain how it's not retarded and lazily written.