Tactic advice about "finding" units
1) one of the best skills is RESERVE.
You can load the resevse with as many soldiers as you want (just buy them, move them to reseve and buy them again....and so on).
Now if you loose some guys you can always replenish them from the reseve guys.
2) put one unit in a garrison, get alot of cheap soldiers instead, doesnt matter which, get into an easy fight and start sacrificing them for the shamans. return to the castle and get your units back from the garrison and continue...

Sacrifice seems like one very abusable spell.

I came there after doing the dorf quests, and spoke to the shaman outside before talking to castle owner.

If you talk to castle owner you need to kill his brother (semi hard)
If you talk to shaman you can just kill few creatures (wolfs/pirates/junk like that) (easy)
Fire rain x2= all dead ;P

Remember to ask castle owner to let you in dungeon to kill dragon :D nice xp