Necromancer

the Necromancer is one of the three bosses that spawn in the Catacombs

Behavior
Will follow the player with wall phasing ability, cannot be hit while he's moving.

Strategy
Right after triggering the boss and grabbing his attention, make sure to run to a previously cleared area (or just clear the area around him) wait for a while until he follows you with his Teleport ability, then retreat and repeat.

Try to kill the Summoned Skeletons and Summoned Skeleton Mages as fast as possible. he will just summon more and more leaving you no time to go anywhere near him to land a strike and the Summons will high amounts of damage.

Watch out for the Possessed Bones as they are deceptively lethal. It is possible to jump over the flying bones and quickly re-engage the boss.

Be aware that having Hands, or Zombies around can make this already difficult fight, even tougher. Be sure to clear the area!

If you have Charm, his summons will kill him, so charm him then keep your distance until the charm wears off or he dies.

Champion Variant
The champion variant has a bluish coloration, with the ability to summon an additional type of Skeleton Mage. When the boss dies, it will come back to life after several seconds to fight the player again.

It can summon a Skeleton Mage that casts a Pestilence-like spell. The pestilence will explode on contact with electricity and fire.