[PvK]

You can also deploy small squads forward with a single undead, perhaps one with good MR, away from your other undeads, to draw banishment attempts away from the others. Since banishment strikes like an inaccurate barrage, many castings can be wasted missing a single undead, which would have taken out dozens of zombies from a big group.

A necromancer or Revenant in the front raising skeletons can also do this well, but exposes the caster to danger.

Command units can hold anti-magic charms, etc.

Higher-level black priests can raise more and more powerful undead - see the manual's tables.
Wights and banes and those hounds with the bare skulls, and shades, are all somewhat tougher undead. The faster of those, and the Undead cavalry are great at the flank/rear attacks to go kill the priests, as Saxon mentioned.

PvK