The paper documents, based mainly on [3]-[6] published papers where a consistent mathematical description of cyberspace and various types of Cyber-Attacks and protection measures are presented, a holistic mathematical approach to a rigorous description of Advanced Persistent Threat (APT) actors’ modus operandi through various scenarios and Cyber Kill Chain stages [2]. After referring [6] to the various elements of Cyber-Attacks we propose some techniques (via 5 scenarios) of tracking the modus operandi of the most sophisticated and non-linear cyber actors, the Advanced Persistent Threat actors that are usually nation-state or nation-state backed and usually stay undetected for an extended time in later stages of Cyber Kill Chain in defenders’ networks. Keywords: Valuation of cyber assets, vulnerability of cyber assets, node supervision, sophistication of an attack germ of cyber-attack, cyber defense, proactive cyber protection, Advanced Persistent Threat (APT) actors, Indication of Compromise (IOC), Tactics, Techniques and Procedures (TTPs).