What mixed hero class of the (Necropolis) faction do you like?

Like it or not, you need to take a combat-focused hero. It significantly increases the hero's survivability and allows you to clear the map alone, especially when playing on Champion difficulty. Having a combat-focused hero is essential to withstand more than one hit from enemies and have time to drink potions.

Otherwise, the Demonologist is decent. You can level up a Reaver, not for the class bonus, but for the bonuses from the tactics (although it's better to take a Knight (of Death) as your second hero).