追ってきて反撃する敵

using UnityEngine;

[RequireComponent(typeof(CharacterController))]
public class EnemyBrain : MonoBehaviour
{
    public Transform target;          // the Knight
    public float aggroRange = 9f;
    public float attackRange = 1.8f;  // stop and swing
    public float speed = 2.6f;
    public float damage = 12f;
    public float attackEvery = 1.2f;

    CharacterController cc;
    float nextAttack;

    void Awake() => cc = GetComponent<CharacterController>();

    void Update()
    {
        if (target == null) return;

        // Flatten Y: a target standing on a block is still "close" horizontally.
        Vector3 flat = target.position - transform.position;
        flat.y = 0f;
        float distance = flat.magnitude;

        if (distance > aggroRange) return;              // Patrol: do nothing yet

        transform.rotation = Quaternion.LookRotation(flat);

        if (distance > attackRange)                     // Chase
        {
            cc.Move(flat.normalized * speed * Time.deltaTime);
        }
        else if (Time.time >= nextAttack)               // Attack
        {
            nextAttack = Time.time + attackEvery;
            target.GetComponent<CharacterStats>().TakeDamage(damage);
        }

        cc.Move(Vector3.down * 9.8f * Time.deltaTime);  // stay on the floor
    }
}

CharacterStats stats;

void Awake()
{
    cc = GetComponent<CharacterController>();
    stats = GetComponent<CharacterStats>();

    // The Knight has CharacterStats too, so that script must never destroy
    // anything itself. The enemy listens for its own death instead.
    stats.OnDied += () => Destroy(gameObject);
}

[Header("Cover & height")]
public LayerMask obstacles;          // the Obstacle layer: blocks, walls
public float reachHeight = 1.2f;     // how far above itself it can still swing
public Transform eye;                // an empty at head height

// A straight line from its eye to your chest. Anything on the Obstacle
// layer in the way means it has no shot.
bool CanSee()
{
    Vector3 chest = target.position + Vector3.up;
    return !Physics.Linecast(eye.position, chest, obstacles);
}

// You standing on a block are out of swinging range, even if the
// flattened distance says you are right next to it.
bool CanReach()
{
    return target.position.y - transform.position.y <= reachHeight;
}

else if (Time.time >= nextAttack && CanReach() && CanSee())
{
    nextAttack = Time.time + attackEvery;
    target.GetComponent<CharacterStats>().TakeDamage(damage);
}

using UnityEngine;
using UnityEngine.AI;                       // NavMeshAgent lives here

[RequireComponent(typeof(NavMeshAgent))]
public class EnemyBrain : MonoBehaviour
{
    public Transform target;
    public float aggroRange = 9f;
    public float attackRange = 1.8f;
    public float speed = 2.6f;

    NavMeshAgent agent;

    void Awake()
    {
        agent = GetComponent<NavMeshAgent>();
        agent.speed = speed;
        agent.stoppingDistance = attackRange * 0.9f;   // stop before it hugs you
    }

    void Update()
    {
        if (target == null) return;

        Vector3 flat = target.position - transform.position;
        flat.y = 0f;
        float distance = flat.magnitude;

        if (distance > aggroRange)
        {
            agent.ResetPath();                         // lost interest
            return;
        }

        if (distance > attackRange)
            agent.SetDestination(target.position);     // it works out the route
        else
            agent.ResetPath();                         // close enough: stand and swing
    }
}