Necromancers, Demonologists, and Shadowlords, are mainly support DPS. They are dangerous in the sense that they are more tanky than Wizards, and that they can debuff you till you can be blown on and die. Later in game, they become more major DPS. Skilled enough Demonologists can exceed the magic attack of even an Archmage.
Wizards, Archmage's, and Avatars, are glass cannons. They have strength in their strong elemental magic which is used in bursts to quicky disable an enemy. Wizards generally are more powerful in the sense of attacking, but they lack defense. At Archmage, this changes. They have more damage reduction.
I'm probably being a bit biased, but I like Necromancers more than I do Wizards.
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