Correction

Assets/Scripts/Game/EnemyDeathManager.cs

+using System.Collections;
+using System.Collections.Generic;
+using UnityEngine;
+
+public class EnemyDeathManager : DeathManager
+{
+    public EnemyFactory enemyFactory;
+
+    [SerializeField]
+    int enemyDeathScore;
+
+    public override void Die()
+    {
+        base.Die();
+        ScoreManager.Instance.IncreaseScore(enemyDeathScore);
+        enemyFactory.Release(this.gameObject);
+    }
+}

Assets/Scripts/Game/EnemyDeathManager.cs.meta

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Assets/Scripts/Game/EnemyFactory.cs

+using System.Collections;
+using System.Collections.Generic;
+using UnityEngine;
+
+public class EnemyFactory : MonoBehaviour
+{
+    public enum EnemyType
+    {
+        normalEnemy = 0,
+        zigzagEnemy = 1
+    }
+
+    [SerializeField]
+    GameObject[] enemyPrefabs = new GameObject[2];
+
+    List<GameObject>[] enemyStock = new List<GameObject>[2];
+
+    private void Start()
+    {
+        enemyStock[0] = new List<GameObject>();
+        enemyStock[1] = new List<GameObject>();
+    }
+
+    public GameObject GetEnemy(EnemyType enemyType)
+    {
+        GameObject newEnemy;
+        if (enemyStock[(int)enemyType].Count == 0)
+        {
+            newEnemy = Instantiate(enemyPrefabs[(int)enemyType]);
+            BaseEnemyManager enemyManager = newEnemy.GetComponent<BaseEnemyManager>();
+            if(enemyManager)
+            {
+                enemyManager.enemyType = enemyType;
+                enemyManager.enemyFactory = this;
+            }
+            EnemyDeathManager enemyDeathManager = newEnemy.GetComponent<EnemyDeathManager>();
+            if(enemyDeathManager)
+            {
+                enemyDeathManager.enemyFactory = this;
+            }
+        }
+        else
+        {
+            newEnemy = enemyStock[(int)enemyType][0];
+            enemyStock[(int)enemyType].RemoveAt(0);
+            newEnemy.SetActive(true);
+        }
+        return newEnemy;
+    }
+
+    public void Release(GameObject enemy)
+    {
+        enemy.SetActive(false);
+        HealthManager healthManager = enemy.GetComponent<HealthManager>();
+        if (healthManager)
+        {
+            healthManager.currentHealth = healthManager.maxHealth;
+        }
+        BaseEnemyManager enemyManager = enemy.GetComponent<BaseEnemyManager>();
+        if (enemyManager)
+        {
+            enemyStock[(int)enemyManager.enemyType].Add(enemy);
+        }
+    }
+}

Assets/Scripts/Game/EnemyFactory.cs.meta

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Assets/Scripts/Game/ImmunityManager.cs

+using System.Collections;
+using System.Collections.Generic;
+using UnityEngine;
+
+public class ImmunityManager : MonoBehaviour
+{
+    static public ImmunityManager Instance { get; private set; }
+
+    public bool isImmune { get; private set; }
+
+    [SerializeField]
+    SpriteRenderer spriteRenderer;
+
+    [SerializeField]
+    float immunityTime = 0.5f;
+
+    Color standardColor;
+
+    [SerializeField]
+    Color immunityColor;
+
+    float immunityStart = float.NegativeInfinity;
+
+	// Use this for initialization
+	void Start ()
+    {
+        Instance = this;
+        if(spriteRenderer)
+        {
+            standardColor = spriteRenderer.color;
+        }
+    }
+
+    public void BecomeImmune()
+    {
+        if(!isImmune)
+        {
+            isImmune = true;
+            immunityStart = Time.time;
+            if(spriteRenderer)
+            {
+                spriteRenderer.color = immunityColor;
+            }
+        }
+    }
+	
+	// Update is called once per frame
+	void Update ()
+    {
+		if(isImmune && Time.time - immunityStart > immunityTime)
+        {
+            isImmune = false;
+            if(spriteRenderer)
+            {
+                spriteRenderer.color = standardColor;
+            }
+        }
+	}
+}

Assets/Scripts/Game/ImmunityManager.cs.meta

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Assets/Scripts/Game/ScoreManager.cs

+using System.Collections;
+using System.Collections.Generic;
+using UnityEngine;
+
+public class ScoreManager : MonoBehaviour
+{
+    static public ScoreManager Instance { get; private set; }
+
+    int score = 0;
+
+    public int GetScore()
+    {
+        return score;
+    }
+	
+	void Start ()
+    {
+        Instance = this;
+        score = 0;
+	}
+	
+	public void IncreaseScore(int i)
+    {
+        score += i;
+    }
+
+    public override string ToString()
+    {
+        return score.ToString();
+    }
+}

Assets/Scripts/Game/ScoreManager.cs.meta

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Assets/Scripts/Game/ScoreMemory.cs

+using System.Collections;
+using System.Collections.Generic;
+using UnityEngine;
+
+public class ScoreMemory : MonoBehaviour
+{
+    public static ScoreMemory Instance { get; private set; }
+
+    public int savedScore = 0;
+
+	// Use this for initialization
+	void Start ()
+    {
+		if(Instance)
+        {
+            Destroy(this.gameObject);
+            return;
+        }
+        Instance = this;
+        DontDestroyOnLoad(this.gameObject);
+	}
+}

Assets/Scripts/Game/ScoreMemory.cs.meta

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Assets/Scripts/Game/ScorePrinter.cs

+using System.Collections;
+using System.Collections.Generic;
+using UnityEngine;
+
+public class ScorePrinter : MonoBehaviour
+{
+    [SerializeField]
+    UnityEngine.UI.Text scoreText;
+	
+	// Update is called once per frame
+	void Update ()
+    {
+        scoreText.text = "Score : " + ScoreManager.Instance;
+	}
+}

Assets/Scripts/Game/ScorePrinter.cs.meta

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Assets/Scripts/Menus/GameOverScorePrinter.cs

+using System.Collections;
+using System.Collections.Generic;
+using UnityEngine;
+
+public class GameOverScorePrinter : MonoBehaviour
+{
+    [SerializeField]
+    UnityEngine.UI.Text scoreText;
+
+	// Use this for initialization
+	void Start ()
+    {
+        if(ScoreMemory.Instance)
+        {
+            scoreText.text = "Score : " + ScoreMemory.Instance.savedScore;
+        }
+	}
+}

Assets/Scripts/Menus/GameOverScorePrinter.cs.meta

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