The most important thing to do when fighting undead is to use the right damage types against the right enemies. Ghosts have 50% physical resistance, skeletons and bone dragons are resistant to projectiles, etc. Typically the ghosts are the highest priority for me because they have high initiative (cursed ghosts move before my royal snakes so I can't cast first) and they have movement 4. I can usually trip up vampires with a death totem, and the bone skeletons die in droves after the ghosts are gone. So a generic undead fight (abandoned castle for instance) goes like this:
Turn 1:
Ghosts rush me, I have my royal snakes wait. I put Holy Anger on my beholders and they blast cursed ghosts, or the biggest stack of ghosts or skeleton archers if there are none. My shamans put a death totem in the back to waste the vampire/skeleton turns. Royal thorns summon a stack of thorns, I put Mana Spring on them, and then my royal snakes lunge at the cursed ghosts.
Turn 2: The cursed ghosts will either attack the summoned thorns (I get my mana back) or my inquisitors and I'll lose a few but the cursed ghosts will nearly kill themselves. Royal snakes attack the cursed ghosts in such a way that the regular ghosts are forced to attack the summoned thorn stack. If that's not possible, I have my shamans put a totem of life down so that the summoned thorns + the totem block the ghosts from getting to the snakes. Inquisitors and Beholders hit the skeleton archers. The battle is all but won at this point. The vampires are 2 turns away from hitting anything because they went back to kill the death totem, there's maybe a stack of ghosts and 1 or 2 crippled stacks of skeleton archers.
You can use the healing spell for direct damage against undead. It has a very efficient damage to mana ratio. If you can't take out a stack of cursed ghosts halfway through the 2nd turn, the enemies might just be a little too tough for your current leadership level. There are also some undead specific artifacts such as the silver rapier (+30% damage to undead) and silver chain )+1 intellect, -2 defense to undead). I found 2 silver rapiers in the Freedom Isles, I can't wait to marry the demoness so I can wield them at the same time.
