To save a dying donkey tail succulent, you should identify the cause of the problem and take appropriate action. If the plant is overwatered, you should stop watering it immediately and let the soil dry out completely before watering it again. If the plant is underwatered, you should water it more frequently and make sure the soil is moist but not waterlogged. Other common causes of a dying plant include pests, diseases, and nutrient deficiencies. You can also try repotting the plant in fresh soil and reducing the amount of direct sunlight it receives.