You say "we have never seen anything about why he would be great?" But he is already responsible of besting Ouki strategically and tactically by leading him into an ambush with the one man that could kill him, then in the coalition war Riboku almost won by sneaking past everyone, only the surprise appearance of Yotanwa prevented him from doing so, then he almost defeated Ousen but our MC and a couple other wild cards prevented it, and in this last part of this war up to the moment when he got tricked into this ambush by Kanki, Riboku had succeeded in destroying almost the totality of Kanki's army and locking Ousen in the south. Again, he is a strategist, one that has mastered Sun Tzu's Art of war, but his true strength is in the talented people he has managed to put under his command, and his only weakness has constantly been the wild and reckless instinctual leaders like Shin. He is the current BBG by the mere fact that he is responsible for most of the suffering and character's growth of our protagonist (Ouki and Duke Hyou died in battles plotted and controlled by Riboku), he has been the mastermind of several critical battles and wars.
"Nothing" has happened in the sense that these two sides have been locked in this battle for "many" (not that many) chapters. But a bunch of things have happened already. We have seen all the subordinates from both sides try their own chance at killing/protecting Riboku; if you have been paying attention you'll see how all of them are accumulating injuries and their soldiers have mixed to the point that this has become an almost disorganized melee, a desperate struggle. Riboku's subordinates barely winning at the moment since they have achieved their objective: to hold up until reinforcements arrive. I would understand if you say that this is frustrating or stressful, because that is what this situation is supposed to be. But boring? You and me have very different ideas of what boring is.
Besides, after all that I have said, don't get me wrong, I want Riboku dead as much as anyone else. If the manga tries to be more historically accurate he will die soon after this defeat (he will lose one way or another), or it could take its liberty (which has happened already) and have him die as direct consequence of this battle.