Sorry, but your screenshots didn’t help much – i couldn’t even see the ones in the second link.
It’s not clear how many men – and how well equipped – you have.
But, by the pictures i could see, you were completely overwhelmed. So, i will try to give general tips for a beginner, and how to deal with the undead.
1 – Equip all your men with the best armor avaliable, as long as it doesn’t hinder their fatigue too much. Your archers are wearing the worst armor in the game, you must have something better. And don’t let your archers be engaged at melee if you can! Massed Wiedergangers are great targets, you hit even when you miss!
2 – At the beginning, it’s crucial to estabilish a line of shield-bearing brothers (with spears to make a spearwall too, early on). It doesn’t work against all oponents, but it does against the undead, except… When there’s a necromancer present, you really should rush to kill him, as you’ve surmissed already. Still, you must do it smartly: kill him with your archers if they are any good; use your line to occupy the ‘gangers while you send a competent man to wipe out the necro; blitz with the entire line to make way for someone or to allow the archers a closer shot…
But that all depends if you simply have the manpower needed for such a task. If you have few and underequipped men, don’t bother, go search for some thugs.
3 – Besides the necromancer, focus on the polearm-wielding Wiedergangers with your archers, as they will probably do the biggest damage; then focus on the ones with the best weapons (though i’m not sure you can recognize them already).
4 – The Wiedergangers have low Action Points, and a spearwall may help on holding them off, and then focusing on the ones that make it to the line. Polearm-wielding brothers (pitchforks at the beginning, usually) provide a real good punch with their weapons, they are some of my best killers.
5 – Don’t advance your line haphazardly over fallen ‘gangers if you haven’t put them down for good – they will disrupt your formation, and may isolate a brother, who will be easily picked off.
6 – At the beginning, i usually hire a lot of cheap brothers and focus on giving them shields and the 30/35 armors.
7 – One of the biggest problem facing large numbers of undead is fatigue. Don’t push your men too hard, or they might be too tired when you need just one strike to save a wounded brother.
8 – Take care of your Companions (the 3 starters), they are amazing. Take care of your archers, it’s hard to get decent skilled ranged fighters.
9 – Remember that every fallen brother means a new Wiederganger for the enemy – shields up! But controlling the fatigue, of course.