Play against Necropolis.

A very large map for a long game, without any secondary settlements? It's certainly possible to create such a map, but what's the point of playing it? Just accumulating resources for 2-3 months and then having a final battle? In this scenario, the Necromancer is definitely doomed, as the Barbarian's higher-level units are quite powerful. But it would be a strange game. In this situation, there should be 6-8 secondary settlements, at least, because there are also computer-controlled factions. You need to capture several of them.

with all sorts of bonuses to bloodlust and initiative

You're talking about an initiative bonus. With mass slowing and Banshee's Cry, even with Dampening, you still have 63% initiative. And if some Daedra finds only 2 artifacts from the racial set, you'll be left with 54%, plus -3 to Luck and Morale.

no matter how many vampires you have

Even with an average number of secondary settlements, there can easily be 200 or more of them after 2 months. So, calculate how much money you'll need to kill them properly when they have a shield with 1500 HP, and they drain more health than their own damage. And the Necromancer will also have more resources. The Necromancer needs to be killed earlier, before he converts a large additional army, which is quite possible on almost any map.