What skills should Necromancers take when facing Orcs on a very large map?

Major Whirlwind and Player Undead don't necessarily need to focus on Darkness to defeat the Orcs, since Undead are not strong in either defense or attack, but there are many of them, so it's better to take Summoning and Light.

I don't understand the logic.
The logic is simple: since Undead are weak in defense and attack, it's better to use their numbers, letting them take the hits and then punish them with Divine Retribution.

Mines will disrupt their Rage.

Mines are a terrible thing against Orcs. They mostly have Wyverns and only one ranged unit (Cyclops don't count, they shoot poorly). But once the Orcs get close, they become useless.
Hmm... it's hard to use mines close to yourself, but what about crystals?

Divine Retribution will do the rest.

No, unfortunately, it won't. Retribution, as you can see, only attacks one stack at a time. Not to mention that the spell itself is masochistic. You have to take losses for it to become effective. And the Orcs will crush the Undead easily. Neither Haste nor Retribution will help here.
Considering that Undead is the most numerous race in the game, they have plenty to sacrifice.

You'll put a Berserker on a Cyclops.

Why on a Cyclops? I'd rather control Executioners; consider one attacking stack already neutralized, and Orcish units rarely reach you (and if the Necromancer upgrades Tactics, they definitely won't). You can use Executioners to attack Centaurs or Cyclops. It's not ideal, but Shamans aren't that scary. Goblins are a different story, but still, the chance of them deflecting a spell isn't very high.
I was responding to post 53. And you're very wrong about Goblins; if there are a lot of them, and they have 2 Knowledge, they can block any mage.

You're saying funny things; most likely, you've only played against the computer and never against a real player on a large map against the Orcs.

There's no need to make such harsh and unfounded statements.
Why unfounded? If you read the advice, you'll see that the most they've done is play against the computer and never against a real player