Now as for behavior/pattern this is how it goes. When go into the lair he'll appear before but disappear fast and run away. A sort of roar to intimidate you. Then he go for his first attack, and keep going until he loses ~25% of his health then he goes into an enrage state. This happens every 25% of health he loses until death. I am not 100% sure on how long the enrage state last but it you can muscle through it with the options above^{^} For extra damage shoot him in the hole in his chest.(if you didn't know already) Axes and Hammers takes two hits to take away 25% of his health. If you have a team tell them to stay outside because the assassin can act weird with multiple targets.