Enemy's health wont get set from different script.

I’m trying to access the health variable from an EnemyMovement class and apply damage/subtract from that health through a different class.

using UnityEngine;
using System.Collections;

public class hitMarker

: MonoBehaviour {

	public EnemyMovement enemyMovement;
	int enemyHealth;

	void Start(){
		enemyHealth = enemyMovement.health;

	void OnCollisionEnter2D(Collision2D collision){
		if (collision.gameObject.tag == "enemy") {
			//Debug.Log ("hit");
			enemyHealth -= DonnyMovement.damageKick;
			enemyMovement.health = enemyHealth;

I believe my problem is the health variable isn’t getting set to the new damage version of it.

Looks like your variable enemyMovement is not assigned. What line give you that error?

If the enemyMovement script is on enemy GameObject you should do like this

if (collision.gameObject.tag == "enemy") {
   enemyMovement = collision.gameObejct.GetComponent<EnemyMovement>();       
  //Debug.Log ("hit");
            enemyMovement.health -= DonnyMovement.damageKick;

Otherwise it makes no sense !