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1. I don't even know which goblins are better. Shamans have a small chance to silence a spell, and with the necromancer's incredible magic, it's practically non-existent. And the trapper isn't very useful either; you can't set a trap on a dragon, ghost, or vampire, and it's almost pointless against zombies, and obviously against liches too. Perhaps against Death Messengers, but the chance is small, and against altar skeletons (perfect, but it's not a guarantee that they'll be there).
2. I always choose combat centaurs. And I would choose them against a necromancer too.
3. In this case, I usually choose warmongers. It has already been described here what they can do with high fury.
4. In my opinion, it doesn't matter with the shamans. Choose whichever ones you like better. IMHO, both grades are equally effective against the undead.
5. I haven't decided which one is better specifically against the undead. But I would probably choose chieftains. Because executioners, under control or berserk, can mow down many of their own. And their morale penalty is useless against the undead.
6. There's no need to think about it. Paoka and that's it. Poison doesn't affect the undead.
7. I've already commented on this.