Besieging undead armies

[Mightypeon]

It may perhaps be worth it for Rose Ulm, who have a handy onbattle caster of solar brilliance, and whose troops have MR14 base with an easily accessable antimagic boost, combined with not having very good amounts of banishment.
Although for Ulm, Iron Blizzard tends to kill more undead than banhisment, and does not care about MR.

[krpeters]

Don't forget the Domkill option. Park an army to defend the territory and some priests to preach. Use your other forces to mop up his other territories, and he'll go away when his dominion vanishes.

[Peter Ebbesen]

Battlefield-wide elemental damage spells and Rain of Stones are other favourites once you have breached the walls if the enemy has thousands of weak undead.