Heroes of sword or magic?

A Warrior can move without an army, but is it worth doing?

It is. In this case, money is spent on potions, stat boosters, and so on.

Necromancer forgot, actually one of my friends really likes to summon 1k skeletons at a time.

Necromancers need corpses; they can't create skeletons out of thin air.

The task is to immediately take down the mage, then it will be easy to deal with the army.

Further attacks – no chance. They will close the gap without any loopholes. You can try in melee, but you need to get very lucky.

Added 2 minutes later

And what about Solmyr? Are there no other mages? And in general, I play as Necromancer, and there mages are valued for raising the dead and nothing else.

Necromancy in the third game is not magic. It is equally effective for Warriors and Mages. You wrote this out of context. Solmyr was just an example. The point is that Dragons have immunity to magic up to a certain threshold, and Black Dragons have complete immunity.