Mujaki is a triggered event. You face him by talking to Etumer in the Gargoyle City in the Plane of Nightmare. Follow his quest text and he will port four groups to a new area to face Mujaki.
Mujaki begins as untargetable. Near him, waves of mobs will begin to spawn usually 3 but sometimes 6 at a time. These mobs will approach Mujaki's platform and if they reach it become untargettable "a servant of Mujaki." Once 36 mobs have been either killed or joined Mujaki on the platform, you can target Mujaki and attempt to take him.