How to move OnTriggerEnter to Update

@willleo10

This should fix your problem

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 
 public class attacking : MonoBehaviour
 {
     public GameObject exampleOne;
     public GameObject exampleTwo;
     public GameObject exampleThree;
     public GameObject exampleFour;
     public Camera camera;
     public Camera camera2;
     private float THEREHealth = 100;
     private float THEREHealth2 = 100;
     private float THEREHealth3 = 100;
     private float THEREHealth4 = 100;
     public Text enemyTEXT;
     public Text enemyTEXT2;
     public Text enemyTEXT3;
     public Text enemyTEXT4;
     private bool PKeyPressed = false;
 
     void Start () 
     {
         camera.enabled = true;
         camera2.enabled = false;
         exampleOne.name = "N1";
         exampleTwo.name = "N2";
         exampleThree.name = "N3";
         exampleFour.name = "N4";
     }
 
     void Update () 
     {
         PKeyPressed = Input.GetKeyDown(KeyCode.P);
         setenemyTEXT  ();
         setenemyTEXT2 ();
         setenemyTEXT3 ();
         setenemyTEXT4 ();
         if (camera2.enabled == false) 
         {
             enemyTEXT.enabled = false;
             enemyTEXT2.enabled = false;
             enemyTEXT3.enabled = false;
             enemyTEXT4.enabled = false;
         }
 
         if (THEREHealth < 0.1)
         {
             THEREHealth = 0;
         }
         if (THEREHealth2 < 0.1)
         {
             THEREHealth2 = 0;
         }
         if (THEREHealth3 < 0.1)
         {
             THEREHealth3 = 0;
         }
         if (THEREHealth4 < 0.1)
         {
             THEREHealth4 = 0;
         }
     }
 
     void OnTriggerEnter(Collider other)
     {
         if (other.gameObject.name == "N1") 
         {
             camera.enabled = false;
             camera2.enabled = true;
             setenemyTEXT ();
             enemyTEXT.enabled = true;
 
             if (PKeyPressed)
             {
                 THEREHealth = THEREHealth - 20;
             }
 
             if (THEREHealth == 0) 
             {
                 camera.enabled = false;
                 camera2.enabled = true;    
                 Destroy (exampleOne);
             }
         }
         if (other.gameObject.name == "N2") 
         {
             camera.enabled = false;
             camera2.enabled = true;
             setenemyTEXT2 ();
             enemyTEXT2.enabled = true;
 
             if (PKeyPressed)
             {
                 THEREHealth2 -= 20;
             }
 
             if (THEREHealth2 == 0) 
             {
                 camera.enabled = false;
                 camera2.enabled = true;    
                 Destroy (exampleTwo);
             }
         }
         if (other.gameObject.name == "N3") 
         {
             camera.enabled = false;
             camera2.enabled = true;
             setenemyTEXT3 ();
             enemyTEXT3.enabled = true;
             if (PKeyPressed)
             {
                 THEREHealth3 -= 20;
             }
             if (THEREHealth3 == 0)
             {
                 camera.enabled = false;
                 camera2.enabled = true;    
                 Destroy (exampleThree);
             }
         }
         if (other.gameObject.name == "N4") 
         {
             camera.enabled = false;
             camera2.enabled = true;
             setenemyTEXT4 ();
             enemyTEXT4.enabled = true;
             if (PKeyPressed)
             {
                 THEREHealth4 -= 20;
             }
             if (THEREHealth == 0) 
             {
                 camera.enabled = false;
                 camera2.enabled = true;    
                 Destroy (exampleFour);
             }
         }
     }
 
     void setenemyTEXT ()
     {
         enemyTEXT.text = "life:" + THEREHealth.ToString ();
     }
     
     void setenemyTEXT2 ()
     {
         enemyTEXT2.text = "life:" + THEREHealth2.ToString ();
     }
 
     void setenemyTEXT3 ()
     {
         enemyTEXT3.text = "life:" + THEREHealth3.ToString ();
     }
     
     void setenemyTEXT4 ()
     {
         enemyTEXT4.text = "life:" + THEREHealth4.ToString ();
     }
 }