So, thoughts for the upcoming War:
Lonkas has made it clear that we're not winning a long, grinding war with the Greenskins on one side and the Undead on the other. Therefore, if we want to win this, we need an overwhelming decisive strike that clears out enough of the Karak to dig in and fortify and turn things into a more "normal" eternal siege that all Karaks go through. On the Greenskin side, this would likely mean finding the biggest and baddest of Greenskin Warbosses and killing them to break the tribes, creating enough chaos that we can secure a firm enough foothold and that by the time the Greenskins finish figuring out who's the new Boss, we'll be too entrenched for anything but a proper WAAAGH to drive out.
On the Undead side, it's a bit more complex. It's commonly said that Undead don't have logistics. In WHF at least, this isn't quite true. Undead don't have conventional logistics of food and water. What they do have is logistics built around there being enough necromantic energy to keep them animated. If there's not enough in the environment (and outside of places like Sylvania or Nehekhara there usually isn't), then the difference has to be made up by the Necromancers and/or vampires commanding them. Konrad von Carstein's army in the 2nd Vampire War in canon literally falls apart when his Necromancers desert him.
This might seem like less of a concern for Nagash, but I'd argue that it very much remains one. Nagash has a lot more necromantic oomph than any of the von Carsteins did, especially while he's hopped up on Waystone power, but he also has a much larger army to maintain, and that army's going through a lot of high-intensity fighting (and if there's not enough ambient magic, then every movement, every sword-swing, has to have the energy supplied by necromancy.) And any magic Nagash uses to keep 100,000 Skeleton Warriors from collapsing due to lack of animating forces is magic he's not using for anything more damaging.
So in terms of how we fight Nagash, I think our focus will likely be as follows:
1) Deny Karak Drazh's Waystone to Nagash, or reclaim it from him if he's already taken it. So long as Nagash doesn't hold the Waystone, he can't use "local" power to maintain his army. This means he has to shoulder the burden himself.
2) Hunt and kill Nagash's Necromancers and Vampires. Necromancers and Vampires are other walking fonts of magical power that can sustain and direct the Undead armies. Each one killed is another restriction on the amount of Undead that Nagash can keep animated within Karak Drazh, or a further reduction in Nagash's personal power as he has to devote his own energies to keeping them all moving and fighting. Necromancers and Vampires are also, it must be noted, much softer targets than Nagash himself.
3) Destroy other unique assets of Nagash's army. Things like cannons imported from the Chaos Dwarfs, or the 'Grumril-khâl' (what I assume to be Undead Dwarfs in Gromril or Blackshard Armor), or his other picked heavy infantry units. These are troops a cut above the average Skeleton, but they also represent material that Nagash cannot replace casually, but will have to make further concessions to the Chaos Dwarfs to replace. They're also the most likely assets Nagash can use beyond raw magical power to force a breach into a Karak, so their loss means Nagash is more likely to abandon a full-out attempt to seize Karak Drazh by storm in favor of a long campaign of wearing the enemy down.
