I just completed it and it was a little tricky, but overall fairly simple to deal with. I didn’t lose any men.
Some tips.
You’ve got two bullhooks to equip your men with, these are pretty OP weapons and are going to be the meat of your damage dealing.
Use your shielded brothers and equip them with spears. Spears are very accurate and deal decent damage. They fail against armor, but most of the Undead in this encounter are unarmored or already half armor, so spears are naturally good against them. Really you just want reliable damage.
I don’t know if the map is the same every time, but for me I started on a small strip of land behind several murky water terrain tiles. Use this to your advantage as any units inside the murky water take significant combat penalties. I set up my shield brothers in a row along there and my bullhooks behind (bullhooks can attack from range 2). I had one sword brother and an archer set to the south. I simply passed turns for several turns waiting for the enemies to come to me, I used all my ammunition dealing the first stragglers that came at me. I DID NOT ADVANCE as I knew the enemies would come to me and I had terrain advantage.
I just waited until the zombies came into my wall and then singled them out, burning them down. The bullhooks can frequently kill one outright or do 80-90% of it’s HP, depending on your damage roll. I prioritized low HP enemies and then enemies with actual weapons. It’s mainly important in the beginning to try and even out the number of attacks you are recieving Vs. dishing out. You always want to try and position yourself such that you can attack and eliminate a threat before it has a chance to attack you back. I think in the entire fight I was attacked about a dozen times and only hit half that.
Once I had the zombies down to more manageable numbers, I transitioned a Bullhook, and the archer + spear brother at the southern flank in towards the necromancer. They made their way there while the necromancer raised and possessed zombies that my main force was dealing with. This southern flank circled around, got into melee with the necromancer and killed him, then moved back towards my main force and flanked the remaining zombies, eliminating them.
Hope this helps!
