Farming for the Ranger emerald for four days now, about 30+ hours, no drop, but....found an interesting pattern on this staff.
The staff drops off a male revenant that seems to regularly pop (maybe 3 out of 4, or five times) directly East of the PH for the MAGI. I have killed him multiple times now and today alone he has dropped the staff four times. my bet is, keep the magi and this revenant PH clear and you will get the staff of elemental mastery; fire.