If you want to play it safe, take down all adds as they arrive and whittle down the Necromancer's HP when you can.

However, if you have a strong enough team, you can leave the far weaker Bone Rabbles up, and hopefully, the Necromancer will then unwittingly fill his ranks with this trash enemy, which you can simply ignore - leaving you to focus on the Necromancer. It's important to get rid of the Bone Solider and Bone Defender adds as soon as you can as they will just make the battle more troublesome. The Vestal and Crusader both have a damage bonus when it comes to Unholy enemies, so they can really help to pack a punch against this boss and his minions. However, even if the Necromancer manages to move positions, the Hellion can attack any position anyway. The Necromancer is resistant to Stun, so it won't always land. The story of Darkest Dungeon is exposed through the narrator, who will give hints on his mysterious past as the player approaches the various bosses. There are two ways to circumvent this: either use the Hellion to Stun him, stopping him from attacking/summoning adds in the first place, or simply use an Occultist or Bounty Hunter to drag him back into position one. The Narrator is a vital part of Darkest Dungeon, accompanying the player in their adventures and creating a dark and stressful atmosphere with his foreboding comments. This will continue until the Necromancer is in position four.

Whenever the Necromancer attacks, he will automatically summon an undead skeleton add that will appear in front of him.

Darkest dungeon wiki necromancer