Destroying assets is not permitted to avoid data loss. If you really want to remove an asset use DestroyImmediate (theObject, true); UnityEngine.Object:Destroy(Object) Gun:Update() (at Assets/Scripts/Gun.cs:75)

I am trying to make a simple fps game, and whenever I hit an "Enemy" with a "Bullet" I always get this error message

 Destroying assets is not permitted to avoid data loss.
 If you really want to remove an asset use DestroyImmediate (theObject, true);
 UnityEngine.Object:Destroy(Object)
 Gun:Update() (at Assets/Scripts/Gun.cs:75)

The enemy is supposed to die after one hit however most of the time it takes 2 hits to kill an enemy and the enemy only dies after the error message is displayed in the console.

The question is: How can I fix this, I don't want the prefab to be destroyed instead I want the version of the prefab I have "copied" many times within the game.

Sorry if this is confusing.

 using UnityEngine;
 using System.Collections;
 
 public class Gun : MonoBehaviour {
 
     public Transform BulletPoint;
     public GameObject Bullet;
     public GameObject Head;
     public GameObject Body;
     public int TotalPistolAmmo;
     public int MaxPistolClip;
     public int CurrentPistolClip;
     public int AmmoReloadPistol;
     public int AbsoluteZeroPistolAmmo;
     public bool CanShoot;
     public bool CanReload;
 
     void Start () 
     {
         Screen.lockCursor = true;
     }
 
     void Update () 
     {
 
         
         if(CurrentPistolClip <= 0)
         {
             CanShoot = false;
         }
 
         if(TotalPistolAmmo <= AbsoluteZeroPistolAmmo)
         {
             CanReload = false;
         }
 
         else
         {
             CanReload = true;
         }
 
         if(Input.GetKeyDown(KeyCode.R) && CanReload == true)
         {
             AmmoReloadPistol = MaxPistolClip - CurrentPistolClip;
 
             if(TotalPistolAmmo - AmmoReloadPistol <= AbsoluteZeroPistolAmmo)
             {
                 AmmoReloadPistol = TotalPistolAmmo;
                 TotalPistolAmmo -= TotalPistolAmmo;
             }
             else
             {
                 TotalPistolAmmo -= AmmoReloadPistol;
             }
 
             CurrentPistolClip += AmmoReloadPistol;
             CanShoot = true;
         }
 
         RaycastHit hit;
 
         Ray mouseRay = new Ray(BulletPoint.position, BulletPoint.forward); 
 
         if(Input.GetMouseButtonDown(0) && CanShoot == true)
         {
             CurrentPistolClip -= 1;
 
             Instantiate(Bullet, BulletPoint.position, BulletPoint.rotation);
 
              if (Physics.Raycast (mouseRay, out hit)) 
             {
                 if(hit.collider.tag == "Body")
                 {
                     Debug.Log("Body");
                     Destroy(Body);
                 }
                 if(hit.collider.tag == "Head")
                 {
                     Debug.Log("Head");
                     Destroy(Head);
                 }
             } 
         }
     }
 
     void OnGUI()
     {
         GUI.Label (new Rect (10, 10, 100, 50), " " + CurrentPistolClip);
         GUI.Label (new Rect (30, 10, 100, 50), " / ");
         GUI.Label (new Rect (40, 10, 100, 50), " " + TotalPistolAmmo);
     }
 }