How to start a boss fight???

Hello, I have a Boss fight which I want to start when my character crosses a specific invisible Box Collider 2D I created on the map. I also created a State animation with default for the Boss to not do anything until it passes. However I do not know how to write the script so that it passes from Default State to actually the entry state of the Boss. I have a base Script:
"

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class EnterTheBoss : MonoBehaviour
{
void OnTriggerStay2D(Collider2D other)
{
HealthPlayer controller = other.GetComponent();

    if (controller != null)
    {
       
    }
}

}

The fight can be triggered in lots of ways…

Some sort of collision/trigger when the player enters a region or enemy radius?

Check the distance between the player and the boss using Vector3.Distance, then trigger something when the player is within a certain range?

When a fight is started in general, call a method to check whether the enemy is a boss. In your enemy script you could use a bool or enum to determine whether it is an enemy or specific enemy type?

I’m not sure exactly what you are trying to achieve, what should happen during a boss fight? Do you want a new fight scene to be loaded, or just trigger some sound effect/visual to alert the player it is a boss etc?