“Ashes’ Will” – “A Sad Journey” [Campaign 2, Mission 2]

Logistics don't seem to be a strong suit for the Necromancers, but that's okay. Maybe Chaos will be useful there? I don't remember who and how we'll have to fight.

Three magic schools are almost never needed. Usually, summoning in campaigns perfectly replaces Chaos: summoning a stack of fire elementals is equivalent to hitting the enemy with earth shock for several turns. And the Swarm spell can неплохо replace various freezing and stunning spells.
Therefore, I agree with the user above. It's a battle to the death. Create seven stacks of ghosts, with two units in each stack, and destroy everything in your path ;) While the enemy is hitting the ghosts, you can easily defeat him with the Avatar of Death, Phoenix, elementals, and dark magic (Berserkers and various summons).