If instead of trying to dispel the darknesses, I try to level the playing field by casting Fog Cloud on top of them, can the Sharrans see through the fog as if it were darkness? The BG3 wiki seems to say so since they can't be blinded.
You can level the playing field by giving your buddies Devil's Sight and the Eversight Ring.
Personally I loved putting half of the enemies into an Hypnotic pattern and letting Gale in the Robe of the Wavemother and a Haste potion deal with the other half.
