Projectile selectively passing through enemies

Greetings,

I am having problems fixing a bug with my isometric 2d action game.

The fireball projectile I have created will hit and kill an enemy, but when there are multiple enemies present, it seems to select only one enemy that it will hit and passes through all other enemies.

When the "selected" enemy dies, it will select a different enemy and pass through all others.

How do I fix this?

The script I am using for the fireball can be found below:

public class FireballController : MonoBehaviour {

 //reference variables for use with damage, collision, and position
 GameObject enemy;
 EnemyHealth enemyHealth;
 Animator anim;
 private Rigidbody2D rb2d;

 public int attackDamage = 100;  //Damage per fireball hit
 public float speed = 30; //initial force added to fireball projectile


  
 void Start ()
 {
     //setup reference variables for damage, collision, and position
    rb2d = GetComponent<Rigidbody2D>();
    rb2d.AddForce(transform.up * speed); //adds initial force to fireball when spawned

    Destroy(gameObject, 5);
    
    anim = GetComponent<Animator>();
 }
 


 void OnTriggerEnter2D(Collider2D other)
 {
     // If the entering collider is the enemy...
     if (other.gameObject == GameObject.FindGameObjectWithTag("Enemy"))
     {
         enemy = GameObject.FindGameObjectWithTag("Enemy");
         enemyHealth = enemy.GetComponent<EnemyHealth>();
         // ... damage the enemy.
         enemyHealth.TakeDamage(attackDamage);

         rb2d.velocity = new Vector2(0, 0);

         anim.SetBool("explode", true);

         Destroy(gameObject);
     }
 }
 

}

Please note that this is my first game in Unity and my first time posting on the forums. I am aware that I have no idea what I'm doing, so helpful comments only please :)