First,
Chaff it with Spirit Hosts or any other unit you believe can hold against it. Necrosphinx or the sorts. Then, go snipe his Wizards with Morghasts, Vargheists, Arcane Bolts, Sepulchral Stalkers, etc. You can bring a Mortis Engine too + Corpse Cart to discourage spellcasting, and even a Liche Priest with his dispel scrolls in a critical moment when you need to attack it with everything you've got.
It has 8 wounds, and a ton of save, but it's not infalible. Some attacks will pierce through and I don't remember if it can be easily healed...
It's certainly difficult, yes, but can be dealt with. Massive Zombie attacks should do the trick as well, since it doesn't matter the attack's quality over quantity, and zombies dish out more consistent base damage than, say Grave Guard (that we want to split armored units in half). Also, try and see if you can use Neferata's Dagger of Jet or the Wight King's Black Axe to bear. Even a single wound inflicted can destroy the monster.
Necrosphinx's decapitating strike is a massive damage output that a 4+ "ward" will not stop on it's full.
But if you really hate the thing, go for Nagash's Hand of Dust spell. That will take care of it

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