The minimum level required for this task is 60.
At that level you can already start hunting necromancers on Yalahar's magician quarter, as the stairs provide you safer escape routes.
The task will be completed a lot faster on Drefia, though. But because of the crowded areas you might consider reaching at least level 80+ to hunt in there with ultimate mana potions, as the manadrain spam can easily outdamage the healing of strong mana potions.