Master of the dark arts, the Necromancer summons undead minions to do their bidding.
Necromancers are relatively rare but very difficult to destroy: not only do they have a solid amount of hit points and a ranged attack that deals True Damage but also the ability to spawn a never-ending swarm of meat shields made of skeletons. The Necromancer raises four to six skeletons at a time, along with an occasional Skeleton Knight. The skeletons spawned do not provide any gold, making them even more obnoxious.
If the Necromancer manages to make a huge horde of skeletons, it will be worth it to use Rain of Fire on it, although it will not give you gold.
Artillery damage both the skeletons and the Necromancer at the same time: two for one kill!
The Tesla x104 is able to hurt all skeletons in its range at once simultaneously with Overcharge, nullifying their advantage in number. Once the Necromancer is exposed, the sheer damage of the bolts can tear through his HP, results in an easy death. However, this needs to be supported well so that it can it doesn't get distracted by the skeletons, as the Chain Lightning can only hit four enemies in one shot.
500mm Big Bertha does not have the ability to harm all skeletons at once like the Tesla, but it can still deal great area damage with all its attacks, capable of shredding huge number of clustered skeletons along with any other enemies that may be passing by.
A Rangers Hideout with Poison Arrows becomes less effective around Necromancers since Skeletons and Skeleton Knights are immune to poison, but this makes the rangers ignore the Skeletons and focus on the Necromancer (or other enemies in range that aren't immune to poison).
Rain of Fire is a good last resort, especially when there is more than one Necromancer on the field and they have a group of their Skeletons with them. The Necromancer is always the priority target; if you use the Rain of Fire just on the Skeletons, more will soon be summoned.
Reinforcements can separate necromancers from their undead. A Necromancer surrounded by skeletons makes it difficult to ensure Reinforcements engage the right targets, so they are best suited as bait. Set them just behind the path of the Necromancer, and they will turn and target them with their magic attacks instead of summoning more skeletons. Reinforcements with the Spear Throw upgrade will help whittle down their HP and provided they are not quickly overwhelmed; an additional reinforcements to engage the Necromancers directly, once the spell has recharged.
The Necromancer can't summon Skeletons while in melee combat. Ideally an advanced player will want to use a melee-capable hero (Gerald, Malik, Oni, etc.) to separate the Necromancer from the Skeletons. The perfect time to do this is right as the Necromancer enters the stage and doesn't have any Skeletons yet. (If the player wasn't prepared, try to keep the Necromancer busy with Reinforcements until the hero can get to them.)