I want to know whether should I make a using UnityEngine statement and inherit from MonoBehaviour in the Monster class to be able to add the script to a game object.
XML Serialization and turning classes into MonoBehaviours are two different things. First, I wouldn’t serialize a MonoBehaviour to XML, usually you only want to save some specific set of data. So maybe you want to serialize a PlayerStats class to XML, but then have a Player MonoBehaviour that lives as a scene object on a component and does things.