He is located on the second floor, and will lead a small handful of Genlock Assassin Acolytes against The Warden. He employs many rogue Talents, favoring those from the assassin and Dual Weapon specialization trees. As in most encounters with Genlock rogues, most if not all of the assailants will be in stealth mode when first encountered though they can still be detected by the sound of their breathing.
It is possible to avoid combat with the Genlock Master Assassin altogether. He is located in a single room with two ogres and a Hurlock Emissary. Both the ogres can be pulled with ranged attacks and then killed in the anteroom. If your party has a stealth character capable of individually defeating the Hurlock Emissary, then you can continue to the Archdemon without the Master Assassin ever revealing himself.