The Chief spawns around -77.74, -37.04, 103,04 . Make sure you clear ALL Ruffian groups in that large cave (roughly 4-5 groups I believe) AND the three summoners that spawn after ruffians.
Spawns as the boss of a ring event on the middle level. Clear all of the ruffians, then the summoners, then the four mobs that the untargetable Chief spawns and you will be able to then kill him.