i want to make my turret attack multiple enemies one by one

My turret just attacks at the first spawn enemy then keep on shooting at the same position even that enemy has been destroyed, I want it to attack other enemies also. My turret works fine with one enemy even if the enemy moves it turns around and destroy it but fails for multiple enemies Here is the code for the bullet public class bulletforblaster : MonoBehaviour {

 public Rigidbody projectile;
 
 public float speed = 20;
 private int counter=0;
 
 
 
 // Use this for initialization
 void Start () {
     
 }
 
 // Update is called once per frame
 void Update () {
     counter ++;
     if (counter > 61) 
     {
         counter = 0;
     }
     if(counter>60)

     {
         Rigidbody instantiatedProjectile = Instantiate(projectile,
                                                        transform.position,
                                                        transform.rotation)
             as Rigidbody;
         
         instantiatedProjectile.velocity = transform.TransformDirection(new Vector3(0, 0,speed));
         
     }
 }

} Here is the code for the turret

 using UnityEngine;
 using System.Collections;
 
 public class Turret_movement : MonoBehaviour {
 
     public Transform Lookatobject;
 
     void Start () {
         
     }
     
     // Update is called once per frame
     void Update () {
         transform.LookAt (Lookatobject);
 
        }
 

Here is the code inserted at the empty object for the bullet

 using UnityEngine;
 using System.Collections;
 
 public class Blaster_destroy_object : MonoBehaviour {
 
 
     
     void OnTriggerEnter(Collider other) {
         switch(other.gameObject.tag)
         {
             
     
         case "enemy":
             DestroyObject(other.gameObject);
             DestroyObject(gameObject);
             GameObject monster3 = (GameObject)Instantiate(Resources.Load("DamageText"),gameObject.transform.position,Quaternion.identity);
             break;
             
         
             
         case "wall":
             DestroyObject(gameObject);  
             break;
         }
         
     }
 }

I am new in the world of unity so if possible explain in detail . Any help is appreciated