how to activate countdown on first Gameobject with tag it being shoot

Hi i wanna start a countdown timer when it have hit my first gameobject, but nomatter what i try i cant get it too work my code look like this

using System.Collections; using System.Collections.Generic; using UnityEngine; using UnityEngine.UI; //Very important!!!

public class CountdownTimer : MonoBehaviour {

 private float timerstart, timeElapsed;
 public Text timerText;
 public int Minutes = 0;
 public int Secounds = 0;
 private float timeleft;

 //public int items; kan bruges til Enemies left
 //private int itemsRemaining; bruges til enemy left



 // Use this for initialization
 void Start ()
 {

     timerText = GetComponent<Text>();
     timeleft = GetInitialTime();

     //Hvormanger fjerner er der tilbage
     //GameObject[] items = GameObject.FindGameObjectsWithTag("Enemy");
     //itemsRemaining = items.Length;

 }

 // Update is called once per frame
 void Update()
 {
     if (enemyDestroy == gameObject.tag"Enemy")
     {
         //Update Countdown Cloak
         timeleft -= Time.deltaTime;
         Minutes = GetLeftMinutes();
         Secounds = GetLeftSeconds();

     }

     //Show Current Clock
     if (timeleft > 0f)
     {
         timerText.text = "Time : " + Minutes + ":" + Secounds.ToString("00");
     }
     else
     {
         //Countdown clock has finished
         timerText.text = "Time : 0:00";
     }
 }
     

private float GetInitialTime() { return Minutes * 60f + Secounds; }

 private int GetLeftMinutes()
 {
     return Mathf.FloorToInt(timeleft / 60f);
 }

 private int GetLeftSeconds()
 {
     return Mathf.FloorToInt(timeleft % 60f);
 }

}