The most cost-effective way to hunt Necromancers by soloing is for knights and paladins to be of level 60+, for mages it is of level 40+.
Mages: The best way to attack them using fire or ice spells, perhaps using runes like GFB and avalanche to help kill their summons. Lower level mages can summon demon skeletons, as it is much safer to make the Necromancer target them. While killing a few Necromancers at once, you should be careful, as they can surround fairly quickly with summons and while hitting you from distance.
Higher level Mages need only hunt these by using Strong Mana Potions, as you'll only need to use Flame Strike and exura. But be careful, if you run into a spawn with 2-3 necros they can still easily trap you with their summons and that could result in a lot of mana being wasted. Try to stand still and use Fire Wave cause they will keep exactly 4 sqm distance from you which makes it easy to defeat up to 5 necros with a few waves.