If you go to
https://wiki.project1999.com/Per-Level_Hunting_Guide, enter "31" and "Necromancer", you'll find a variety of options.
One solid option is the Tower of Frozen Shadow ... but be careful going to the 2nd+ floor, because if you die you'll have to kill the keyholders on the first floor (naked) to get back to your body. Another great (classic) option is to kill the residents of Karana.