Best way to go about killing the undead is equipping a few brothers with millitary cleavers (extremely expensive) or the rough cleavers that you get from killing orcs. The decapitate skill puts enemies down to stay down. If you’ve got someone in your group with the “fatality” stat, you don’t even have to give him a cleaver, any edged weapon ensures a decapitation if I understand the perk properly. If you’ve got a utility unit with Rally, then undead battles where you use cleavers, even the orcicsh ones, become fairly simple. It may be the cheapest option as well, since millitary cleavers are pretty expensive to buy.
Hope this helps!!