[RYTEDR]

Try to use Necro Call on stacks that will yield you a good bit of ghosts. These summoned ghosts ignore leadership requirements. Build them up all you want and they will remain on your side. It is very easy to get a very threatening, game-breaking stack of ghosts if planned correctly, especially in late game: the numbers they spawn when they hit become outrageous, especially if they land a lot of crits. A lot of times you'll spawn more ghosts than lose if you're careful. Using this I can usually conjure myself up a tank that replenishes itself and becomes stronger and stronger as the battle goes on.

A lot of animals summon ghosts when you use Necro Call on them, so I typically use level 1 Call of Nature (or higher depending on how strong the enemy is and if I can risk raising less ghosts in the beginning), send them out on a suicidal run, then use their corpse to summon my ghosts.

EDIT: Level 1 Call of Nature is great because hyenas and wolves can summon from them, which are guaranteed ghost stacks. Unicorns and black unicorns (and also bears. All bears summon ghosts as well) are great if you want cursed ghosts, but you'll have to sacrifice the rather large amount of mana for the offchance that you might accidentally summon a useless stack (like dragonflies).