Custom Collision

CustomCollision.cs

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

#if UNITY_EDITOR
    using UnityEditor;
#endif

public class CustomCollision : MonoBehaviour
{
    /*
    public Vector3 offset;
    public Vector2 size;
    public LayerMask checkLayer;
    */
    [SerializeField]
    Vector3 offset;

    [SerializeField]
    Vector2 size;

    [SerializeField]
    public LayerMask checkLayer;

    bool isFoundEnemy = false;
    Collider2D target = null;

#if UNITY_EDITOR
    void OnDrawGizmosSelected() {
        Gizmos.color = Color.yellow;
        Gizmos.DrawWireCube(transform.position + offset, size);
        Handles.Label(transform.position, "Trigger Area");
    }
#endif

    void Update()
    {
        if (Input.GetKeyDown(KeyCode.G) && isFoundEnemy) {
            //Attack...
        }
    }

    void FixedUpdate()
    {
        target = Physics2D.OverlapBox(
            transform.position + offset,
            size,
            0.0f,
            checkLayer
        );

        isFoundEnemy = (target != null);
    }
}