[Agrajag]

Priests which have water magic can cast quickness on themselves for double efficiency (I think a water-9 blessing would work well in most cases as well).
Anyway, the ultimate undead horde (AE) killer is alteration-7 Bone Grinding, it doesn't say so in the spell description, but since AE undead are... Well... Bones, it grinds them up and kills them all, or atleast that's how I lost a horde of longdead horsemen to an AI once. It is however quite hard to get enough death magic levels on one mage to cast the spell (unless you are Ermor...).