In light of the necromancer's poison immunity, I think they need to either create a way to get past the poison immunity, or create other types of damage that can be inflicted directly. Like Spectral damage. You could use ectoplasm counters to determine how much direct spectral damage a creature receives each upkeep.
Or they can come out with equipment that attacks/deals damage to the wearer each upkeep. We could have bracelets/wristbands (count as gloves) with a flame damage barrier that inflicts a burn on the wearer that never goes away as long as the equipment is worn.
Or a helmet that increases channeling by 1 at the cost of taking 2 damage every upkeep. (Probably mana cost 3 or 4)
Or a choker (type of necklace; counts as amulet) with teeth in it that rolls 1 die critical damage on the wearer every upkeep.
Or an amulet that once per game allows one to summon a single creature of any level as a quick cast, but then lowers the life (not damage) of the mage who summoned that creature by an amount equal to that creature's level every upkeep from then on, even if the amulet is destroyed or otherwise removed from the mage who last used it's effect.
Any other ideas?
On a side note, I also propose that if a mage is killed from lowering their life total rather than dealing them damage, that mage is obliterated. And it would be cool if there was equipment with the hindrance: heavy. The wearer of a heavy equipment would have slow.