How to only allow collision only on bottom side of 2d box collider Not top!

1 yes all 2d game and colliders

2 yes many enemys spawning

3 yes bottom of anywhere on the enemy (bottom)

1 enemy is spawned randomly and moves all over screen

2 so far yes 1 , 2d box collider on enemy and player sword has box collider on edge of sword.

3 no I may do that later but no, rite now no triggers anywhere

4 so far 1, 2d box collider on enemys

I have spawn scripts if needed but to keep things simple heres death script only

 using UnityEngine;
 using System.Collections;
 
 public class fishdedd : $$anonymous$$onoBehaviour {
  //Animation anim;
         public bool isDead = false;        //has the player collided with a wall?
 
          Animator anim;                    //reference to the animator component
 
 
     void Start()
         {
 
             anim = GetComponent<Animator> ();
 
         }
 
         void OnCollisionEnter2D(Collision2D other)
     {
 
         if (other.gameObject.tag == "Player") 
         {
             //Animation.Play("fishded");
             anim.Play("fishded");
             anim.SetTrigger("Die");
             //GetComponent<Animation>().Play ("fishded");
             Destroy (gameObject, 1.3f);
 
             isDead = true;
             GetComponent<AudioSource> ().Play ();
                 
         }
     }
             }