Wispmothers are usually accompanied by three Wisps. So long as the Wisps are alive, they grant the Wispmother boosts to her Health, Magicka, and/or Stamina. So make sure to kill the Wisps before her. She is much weaker without them to play backup. Once you get her health down enough, she splits into two more Wispmothers now known as Shades. Keep track of who is the real one. That will end the battle quickly. Take note that Wispmothers are gifted with Speed Burst, Regeneration, and Waterwalking.