They are nasty, if I rember rightly they skirmish and throw poison orbs, you have to take a toughness test, if you fail you take a hit with no armour save, they can throw in combat.
The best way to deal with them is rocket launcher, Mortar, or charge with knights to do enogh damage to force them to flee.
Watch out for the plauge spell, you must stop that at all costs!
Try to turn a flank then use your knights to exploit it.
Good Luck