This arc shows the limitation of Hara's writing ability for battle scnario.
He needed Ousen to lose big because that was what happened in history but could not come up with a good scenario for it. First Akou took the obvious bait from Riboku throwing whatever grand plan Ousen might had out the window and he could not improvise. When SBS and his army attacked Ousen's army and proved to be stronger than anticipated, instead of ordering a retreat to regroup and cut his his loss, Ousen bet everything on a huge and very risky life/death gamble that not only Souou and Akou would reach him in time but they will be able to beat SBS even though he had no info on SBS's max strength and did not actually have any backup plan in the event they failed.
In short, Ousen lost this big in mere half a day because he acted out of character by taking a gamble when the odds were very much against him with predictably disastrous results.
And through this, Ousen did not even wield his glaive and the only things he had to say were "I win" only to look like an idiot after and concern for Denrimi who had not done anything impressive onscreen.
Bad arc overall. If not for his feats from previous arcs, Ousen would be a fraud. I get that Hara has a limited number of brilliant battle strategies and tactics he could come up with and has to save them for later arcs but come on.