drone to target enemies

I am working on a script for a drone that will be along side the player. It will target multiple enemies and destroy them. Does anyone know of any good tutorials I might learn from? I have a script I have written but it is not working and I am not sure why. I would like to find out why it is not working and how to fix it. Below is the script.

 using UnityEngine;
 using System.Collections;
 
 public class eshooter : MonoBehaviour 
 {
     LineRenderer line;
     public GameObject explosionA;
     public AudioClip boom;
     public GameObject explosionB;
     public GameObject explosionC;
 
 
         // Use this for initialization
     void Start () 
     {
         line = gameObject.GetComponent<LineRenderer>();
         line.enabled = false;
     }
     
     // Update is called once per frame
     void update ()
     {
         Vector3 fwd = transform.TransformDirection(Vector3.forward);
         if (Physics.Raycast(transform.position, fwd, 2500))
     {
             StartCoroutine("FireLaser", 0.25F);
             audio.Play();
                     }
     }
     IEnumerator FireLaser(float lengthOfTime)
     {
         line.enabled = true;
         var t = Time.time;
         
         while(Input.GetButton("Fire1") && Time.time - t < lengthOfTime)
             
         {
             Ray ray = new Ray(transform.position, transform.forward);
             RaycastHit hit;
             
             line.SetPosition(0, ray.origin);
             
             if(Physics.Raycast(ray, out hit, 2500))
             {
                 line.SetPosition(1, hit.point);
                 if  (hit.collider.tag == "Asteroid")
                 {
                     Instantiate(explosionA, hit.point, transform.rotation);
                     Destroy(hit.transform.gameObject); 
                     audio.PlayOneShot(boom, 1F);
                 }
                 else if  (hit.collider.tag == "Missile")
                 {
                     Instantiate(explosionB, hit.point, transform.rotation);
                     Destroy(hit.transform.gameObject); 
                     audio.PlayOneShot(boom, 1F);
                 }
                 else if  (hit.collider.tag == "Ufo")
                 {
                     Instantiate(explosionC, hit.point, transform.rotation);
                     Destroy(hit.transform.gameObject); 
                     audio.PlayOneShot(boom, 1F);
                 }
             }
             else 
                 line.SetPosition(1, ray.GetPoint(2500));
             
             yield return null;
         }
         line.enabled = false;
         
     }
 }