She is very strong in the One Power, equalling Naime who is descibed almost as strong as Lelaine Akashi. However this is not confirmed by TWoTC where Naime's level of strength is described only as 16(4) (and not as 14(2) which is a level below Lelaine).
Anyway Rysael stands in higher rank than Rainyn, who is stronger than her, making clear that, unlike Aes Sedai, among Windfinders the rank is not defined by strength in the One Power.
When Elayne Trakand is captured by the Black Ajah Birgitte Silverbow manages to convince Chanelle din Seran White Shark and the rest of the Sea Folk Windfinders that without Elayne there is no bargain. They Travel to the area where the Black Ajah are riding a wagon with Elayne inside. The strike force ride towards the wagon, but are decimated by Balefire from Asne Zeramene. Birgitte finally convinces the Windfinders to actively attack the Black Ajah. The Windfinders all link with Chanelle in control and attack the wagon with lightning, killing Asne in the process. They shield all the remaining Black Ajah members so they can be taken captive by Elayne's troops.