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

The logic is simple because the undead have weak defense and attack, so it's better to use their numbers, exposing them to attacks, and then punish them with divine retribution.

Considering that the undead are the most numerous race in the game, there are plenty of units to sacrifice.

They may have the biggest stat boosts, but they are far from having high health. As a result, only zombies are "meaty" in their army. If you are gathering creatures from several cities and turning them into undead, then there is nothing to discuss. Such an army will be very difficult to break, and not just by an orc.

Hmm... it's difficult to use mines near yourself, but what about crystals?

Crystals are not cool enough to influence the outcome of the battle. You can surround yourself with mines, but the orc will simply not move from its position and will attack those standing nearby. Then, it will reach the others over the corpses, and if it steps on a couple of mines, it won't be critical. However, in this regard, everything is unpredictable and is determined during the battle.

You are very wrong about goblins. If there are many of them, and they have 2 attack, they will block any mage.

You are the one who is mistaken. If they always blocked in your experience (which I highly doubt), then you were just lucky. The base chance of deflection is 10%. Even if it doubles, it will only be 20%, which is definitely not something you can rely on. There is a much higher chance that the spell will cost twice as much mana or that the casting of the spell will be reduced by half.

Why without foundation? If you read the advice, you will understand that at most, they played with the computer and not with a real player.

What exactly made you say that? And... are you saying that you have a lot of experience playing online, so you only see "funny things" here?