Snuff out Marignon as fast as
inhumanly possible! I don't think you can destroy that site, but you can try to stay there as briefly as possible with actual undead.
If you can summon/hire up things that can't be banished, that would be best. Otherwise, do your unholiest to protect the undead you use, but don't count on any undead "survivors." Scatter some undead in front to attract inefficient banish attempts. Race at their priests with undead riders. Get things that fly, to go fly and attack their priests. Kill the priests and take their lands and destroy their temples, quickly. Blitzdeath!
PvK