2D movement restriction of x and z

I have this script...

 using UnityEngine;
 using System.Collections;
 
 public class AiMovement : MonoBehaviour {
 
     public Transform target;//set target from inspector instead of looking in Update
     public float speed = 3f;
 
 
     void Start () {
 
     }
 
     void Update(){
 
         //rotate to look at the player
         transform.LookAt(target.position);
         transform.Rotate(new Vector3(0,-90,0),Space.Self);//correcting the original rotation
 
 
         //move towards the player
         if (Vector3.Distance(transform.position,target.position)>1f){//move if distance from target is greater than 1
             transform.Translate(new Vector3(speed* Time.deltaTime,0,0) );
         }
 
     }
 
 }

But im having trouble restricting the X and Z rotations. ive tried everything between transform.rotation.z = 0, to adding other scripts in order to remove the rotations but nothing is working. If anyone has any suggestions itd be most helpful. Thanks in advanced :)