The fight being very hard is the point. Goblin cities, orc warcamps, the black monolith are meant to be end-game challenges for companies that have done pretty much everything else.
The shaman-spells do some fatigue to the shaman but not a lot, they can do it very often until they take a break for a turn or two.
Retreating multiple times is a good way to wittle them down, takes forever but works.
Kite-shields+shieldwall, night and, if you have the supporters edition, fangshire on your best archer.
If your archers are good enough (like around 100 ranged skill) they can do a lot of damage even at night.
If the fight is still to tough to crack just leave and get some more veteran levels, the city’s not going anywhere.
I found retreating a couple times to whittle down their numbers worked for me, my mercs are around 13-15 vet lvl. I focused the archers and worgs first and once they were down, the rest came within melee range and got mowed down. Also just finished the Black monolith after it, much harder imo but the loot was worth it kinda…
Still trying to figure out the reqs for this davkul set as its the last armor set id like. I have 2 cultists in formation and 4 in reserve but the only events ive had are conversion & raising the sacrifice guy. Hope it comes eventually.