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.