Dunno about the pretender, but magic-wise try Van sabbath. Van jarls as slaves, vanadrotts as masters. Cast storm and air power, then van jarls proceed with thunder strikes while drotts take turns casting rejuvenation between thunder strikes. It resets fatique for all slaves - result can be really scary. Lots of high precision mages spamming thunder strike and not getting any fatique until turn 5.
One big problem is that sometimes storm is the last spell being cast... thus air power fails, whole combo is screwed. Dont know why it happened for me, really, I thought globals are always first in the stack. So a much safer idea would be a staff of storms or just storm on turn 1, while your sabbath is busy gutting blood slaves.
When I had that untimely storm, my Hangadrotts (I tried Van sabbath with Helheim) decided that spamming raise skeletons is a very good idea and killed Jarl slaves with fatique
At least the huge tide of skeletons won that battle for me.