UnityRotateToObject2

 using UnityEngine;
     using System.Collections;

     public class Test : MonoBehaviour
     {

         public GameObject[] target;
         public float objectDistance;
         public float rotationDamping;

         void Start()
         {

             target = GameObject.FindGameObjectsWithTag("Enemy");     //It will look for all objects tagged as "Enemy"

         }

         void Update()
         {
             for (var i = 0; i < target.Length; i++)
             {
                 objectDistance = Vector3.Distance(target[i].transform.position, transform.position);

                 if (objectDistance < 20f)
                 {
                     lookAtPlayer();
                 }
             }
         }

         void lookAtPlayer()
         {
             for (var i = 0; i < target.Length; i++)
             {
                 Quaternion rotation = Quaternion.LookRotation(target[i].transform.position - transform.position);
                 transform.rotation = Quaternion.Slerp(transform.rotation, rotation, Time.deltaTime * rotationDamping);
             }
         }
     }