How to subtract variable from singleton instance?

I've created a singleton instance called instance in my code above that UnityAnswers won't format right so I'm not going to waste anyone's time trying to put it in here.

I have the following code below and I want to subtract damage from an individual instance's health float but when the enemyDamaged function is called, it seems to subtract damage from every instance's health float. What can I change so it'll only affect the instance I called this function for?

 public static void enemyDamaged (string enemy, float damage)
 {
     instance.sprite = GameObject.Find(enemy).GetComponent<tk2dSprite>();
     SoundManager.PlaySound("hurt");
     instance.shading = instance.sprite.color;
     instance.health -= damage;
     Debug.Log(instance.health);
     instance.flashAmount = 0;
     instance.rend = GameObject.Find(enemy).GetComponent<Renderer>();
     if (instance.health <= 0)
     {
         instance.KillEnemy(GameObject.Find(enemy));
     }
     else
     {
         instance.InvokeRepeating("Flashing", 0, 0.05f);
     }
 }