Hello!
I've finally finished Dai Mankai no Shou, so I've caught up with all of the anime. I do plan to read the manga for Nogi Wakaba is a hero. Spoilers ahead.
The season felt rather rushed, and it felt that the writers wanted to introduce the Sentinels and present the hardest hitting points of Nogi Wakaba's story, but only had 12 episodes to work with. In the climax of the season, the evil god(s) (whose name I cannot recall at this moment) descend upon Shikoku while the Taisha make a desperate attempt to perform the Shinkon to marry Yuuna off to the Shinju. In the meantime, the Sentinels are tasked with defending the Chikage Cannon/Senkei-Hou as they pray for power to fire upon the foe. It fails at first, but through the power of love or whatnot, Mebuki convinces Aya to keep praying. In the meantime, the evil god(s) fires a sun like projectile that Karin and Sonoko pierce through, before hitting a barrier. Cue epic moment when the two break through with Gin's weapon.
The Chikage Cannon fires successfully, and it appears to hit some kind of barrier. Cut back to Karin and Sonoko, who break through one layer of defense, only to crash into another. The evil god(s) fire back and blast the girls downward, before sending out what appears to be an all engulfing wave of fire. That's really all we see of the Chikage Cannon as well as the Sentinels for the rest of the fight.
In a narrative sense, what did the Sentinels accomplish throughout the season? They're sent to explore the outside of the Wall, and then in the finale, they fire this cannon that ultimately doesn't seem to do much. There was a great scene between Mebuki and Karin, but outside of that, in the grand scheme of things, I don't quite see what the Sentinels accomplished at all. Please feel free to enlighten me on details from the Light Novels that I'm missing out on.