Playing as the Stronghold faction (Disciples 2).

He is immune to death, so they will lose.

Well, the Lich Queen or Death Knight will help.

And the tactic with ghosts and undead is for noobs.

It's pure luck here – what kind of units the opponent will develop. Against warriors, undead and specters will be the best, but if they suddenly decide to develop a mage, then liches with vampires are needed.
If there is a Lord of the Dead in the army, then the mage will also have a hard time.

The undead have a warrior-ghost and a templar, as far as I remember.

I remember poorly. The templar is there, but he upgrades to a Lord of the Dead.

Five hundred, if I remember correctly, for the building, and another thousand for the stone-skinned werewolf, which dies from two elemental attacks and is vulnerable to magic.

Still, weapon damage is very common in the Adepts. No one forces you to put the werewolf in the main army. But to put it with banshees, just to scare and beat particularly zealous orc-goblins – please.
Although, IMHO, a Satyr is no worse.

UPDATE. I continue to fill in the gaps in my knowledge. The werewolf is still MUCH more expensive than it should be. I take back my words.