SOP to kill Grendelkin.
If the Grendelkin doesn't have a shield of valour (I bet my hat that is not the standard equipment for grendelkin
) then take one indy commander with Black Bow of Botulf, and another indy commander with Ethereal Crossbow. Script each with Fire at Large target. If you want to be extra sure, bring in a third indy commander (or a nature/airmage who first boosts his precission) with Vine Bow.
If the Grendelkin is not 100% frost resistant. Script your water mages with Frozen Heart.
If the Grendelkin is not 100% shock resistant. Script your air mages with Orb Lighnting. If the Grendelkin is still alive after a couple rounds, send in a militia man to gut the now passed out Grendelkin.
Use an astral mage, scripted with Horror Mark x3, Retreat.