People keep bashing Riboku's strategy but, if you think about it, it's quite sound.
First he took out Hi Shin, a unit which has shown more than capable to wreck or disrupt his plans often by being unpredictable and highly mobile by tying them and the Gyouku Hou in situations where they cannot retreat to provide support (losing a flank is a disaster, and Riboku is too much of a threat to ignore). The left flank with Yotanwa is also swamped on their own battle, so there's that too.
In the center, if you remember, there was a full advance in Zhou's side, which means all units would get into battle sooner or later (that's why Shibashou seemed to appear out of nowhere, he was just behind the other units and had just arrived). With the different Qin units tied in attrition battles (probably their worst situations while being in numerical disadvantage), this means that there's a road directly to Ousen's whittled unit for Shibashou to use unimpeded and attack him (and meaning that Shibashou looks more like a "combat" general instead of a "strategist" general, it's not without basis). Simply put, Zhou's units has to hold long enough for Shibashou to kill Ousen. But it's not without risks.
The first one, is Shin taking Riboku out. But there's the catch: it's a siege battle. Those things require preparations and even then they're not Shin's forte. It will cost Shin time, which is what Riboku wants. Another is Youtanwa winning in her flank and going to the help of the centre, but her match is a hard one (strategist + knowledge of her tribes + grudge). Second-to-last is one of Qin's center units winning/breaking off their fights and going to provide support to Ousen or the other units, but I guess Riboku thought about it and put bad match-ups for them.
But the biggest one is that, ultimately, the strategy IS simple. It's not an incredibly convoluted plan with multiple layers of deception (this time, maybe one or two at most), but a relatively simple one which relies in putting Ousen in a checkmate while nobody is able to do anything to avoid it. But somebody who's used to being crafty, making plans, and generally being a magnificent bastard and had the time and position to see the big picture would probably had more than enough time to see something like that coming, see where it fails and plan countermeasures for it.
Somebody like, y'know, Ousen.
TL;RD: Riboku's plan is solid despite what everyone thinks, but simple. Ousen is too clever to fall for it without a plan on his own.