Depends what sort of Necro you're talking about. Dexxer based or Caster based. Both can have a lot of power if setup right.
As a caster you would generally be using Wraith Form as your damage leeches back mana and assuming what you're fighting has a big or unlimited mana pool (some things do), then you can spam big spells until it's dead. Helped enormousely of course by there being no Spell Damage Increase cap in PvM.
As a Necro Dexxer, you would generally be looking at playing in Wraith Form with Curse Weapon, or Vamp Form. Both have different strengths and weaknesses. It is possible to be setup to run either. Being a Dexxer then throws up ranged or non ranged and again affects how you play the character and what armor you use.
As a rough example of what is capable:
A high SDI caster with Slayer spellbooks on a Necro/Mage can output 700+ damage in a spell (well a combo such as Flame Strike Omen, on a Corpse Skinned target), but at the disadvantage of (if in Wraith) being on foot) and unable to outrun certain things, equiplastweapon macro becomes your friend switching back and forth between Mage Weapon and Slayer spellbook when in trouble for melee defense.
The highest damage output on a dexxer is (or was, they keep monkeying with base damage from what I can tell) around 273, which if they've fixed certain weapons could be every 1.25s.
If talking about Doom specifically, on a caster although you can still do the big damage, on the Dark Fathers you're going to want to hit and run mounted rather than Wraith Form, so although you hit the high number it's not as regular as if something is tanking and you can just sit and spam an fs/omen macro. A dexxer on the other hand is much more consistant damage but you have to be conscious of other factors... such as, being Blood Oathed, the various Undead the Dark Father will spawn (in regards to using Enemy Of One, or certain Slayers make you take additional damage)
Generally people play Sampires (Vamp Form, no Spirit Speak, Samurai), which are effective but so epically boring and run of the mill now that no one does anything inventive with them. There are a lot of variations of Vamp Form dexxers possible, Evasion with Parry, with Resist, with bandages, with Discord or other Bard skills/masterys, my favourite although I've not played it yet is Ninja based and uses Mirror Images.