Three groups of NPCs:
1. Various beasts spawning from two sides of arena or more (for those to level 15)
2. Organised group of Golems (story is they are breed by Sorcere for such prupose for 15+ levels)
3. Duergar merc group


4. OPTIONAL: Rifts opened by mages to spawn something outer-planar

Also is important that beasts are "fainted", golems can be destroyed mages can make new lol, and duergars(can be of each race) get spared-arena is their work, they can be NPCs around it also.

Easiest to solve this for programers would be that district leaders are given some kind of spawn and kill switch that is on Kitty server, that would work similar but limited

Well this is my imagination lol.