Kahglia wasn't impervious to the cloud of death lingering around the Flesh Golem, but she was a lot more resistant to it than Toz. Not to mention how she didn't have to subject her entire body to the cloud of death when fighting the Flesh Golem. Since the cloud formed a fluffy layer around the Flesh Golem, she only had to push her hands and forearms through the spell to hit the Flesh Golem.
But since the cloud boosted the Flesh Golem's strength and, most of all, its regenerative capacity, Kahglia felt that it was a little futile even trying to attack the Flesh Golem. She would have considered attacking the lich since it was the one who had made the cloud. But after casting the spell, the lich didn't control it anymore. So, removing the lich would only prevent anything similar from happening again. And it wasn't even certain that the lich had any more tricks up its sleeves.