Don't get me wrong, but I have the impression, that not everything here is played as it is supposed to.
1.) First of all, how are the Mortans able to get spells with 20+ range? The maximum in their list is 10". Even with 1 Ancestral Token, that's only 12". There is no long range for spells. When they channel through the Legio Mortum models, the direct damage is not reduced by their faction ability.
2.) In order to get the good undead units, the Mortifex/Necromagus has to generate either Ancestral Tokens or personally kill enemy models. To do so, he has to invest ACT in spells or risk enganging enemies. So, less ACT to generate Legio Mortum and/or he has to stay behind his troops while they move forward. But then they lose the boni from his Ancestral Tokens. Also, it shouldn't be possible to create more than 4 Legio Mortum per turn with 3 necromancers as it's 3 ACT per spell attempt (2 from the Mortifex and 1 each from the Necromagi).
3.) Additionally, he has to use 1 ACT for the Bodyguard tactic or if you are further away for the Spirit Shroud tactic (and he can't have both at the same time). Remember, he can't use Bodyguard as reaction - the tactic has to be on him before he is attacked!
4.) Remember, that for thaumaturgy spells ( = offensive spells against enemy models), the spell complexitiy is raised by the target's MAG value.
5.) What about your Mongrels paired with a Fallow Shaman with the Bulging Biceps spell? With this spell + powerful fighting style (or charge bonus or upgraded spell), you get over the magic POW4 limit and should walk through the Legio Mortum as if it wasn't there.
6.) Have you tried Fomorian Guthackers? Sure, they don't have models, yet, but we here are playing with proxies often not to miss out on all the units. With their Circular Slash, they should be the perfect Legio Mortum killers. Same counts for the other elite high-POW units of the other factions like Hill Ogres, Sons of War, Einherjer (they will have quite an easy game) or big monsters like Cerberos or Fjellgangr.
7.) Your Reaver subfaction can be an early threat for the Magistratum armies with the Raider ability of your Bloodgut Ravagers. Maybe two of them with either Reaver Tribesmen (base POW4) or even Skulleaters (with activated Carnage tactic, they all may make Slay Movement with POW6 attacks). So then you have 2 dangerous units closer to the enemy lines right at the start of the game and so they don't have that much time to generate Legio Mortum or Ancestral Tokens.
8.) You can give your Gorelord the Urghast Charm to make him completely immune to enemy spells and magic attacks - it's only 3 points.
And without enough Ancestral Tokens, the Legio Mortum may make only 1 POW 2 attack with MEL 5 as they normally may not get ACT-tokens from the warlord and may not chose a fighting style. So, they should not be even close to good enough for your GoreLord with Urghast Charm and Gut-Plate of Negirrath for that sweet DEF15 and (almost) immunity to critical hits.
9.) Avoid the enemy troops. The Legio Mortum are slow and the characters need their ACT to raise troopers - they will have a hard time to get to you. After some games, your opponent will realize that you (especially the fast Banebrood) can easily avoid him and so, in the end, nobody wins - it's most likely a boring draw. And then he has to change his list.
That's one of the most common errors in tabletop gaming: People think, they HAVE to engage the enemy troops or fight at all. But remember, that you as Banebrood player have by far the fastest army in this game - nobody but you dictates when and where to fight - at least not as good as you.^^
Soo, I hope these few points can help you to destroy that evil necromantic threat.