Use the Bramblestaff on Three-Tooth and kill her 2 Deviate Crones in the Overgrowth.
According to Gaines, "Three-Tooth" is what the quilboar call their matriarch. She's been twisting the thorns of this crazy jungle to her own ends, and it was her orders that Gaines be "torn apart and fed to the overgrowth." Nice lady.
Let's give her a taste of her own. Take the Bramblestaff you recovered, and use it on Three-Tooth to take her out of the picture. Then kill the crones she's been training, to ensure nobody else can take her place.