The Necromancer is a complex enemy that uses magic and melee attacks to try to kill you. The first time you encounter a Necromancer is going to be while doing the Talk to the Bloodqueen quest and it’s going to be hard to beat unless you have a great armor and weapon for your level.
Necromancers use a frost spell to slow you down and drain your stamina at the same time, if you don’t do anything while they are casting this spell it will probably drain most of your stamina leaving you open to having almost nothing to defend/attack on your abilities side. It also attacks with shock spells that deal quite some damage to your character if you are not expecting them.
How to kill Necromancers?
I found the best way to kill them is using a fast weapon and doing as much damage and combos as possible in a short period of time to prevent them from draining to much of your stamina and health. But to be completely honest, I died the first 4 times I faced a Necromancer.
Necromancers are weak to poison and resist fire and frost damage. So your best chance to kill him is to avoid using fire or frost spells and focus on using melee attacks to bring him down as soon as possible either dealing big blows with a two handed weapon or multiple successive attacks with a small one-handed one.