Well, let's start with the basics. Any hero participating in a battle will benefit from combat skills (so they don't get taken down by the first enemy). The question is when to take this additional skill.
The General bonus affects not only monsters but also heroes, boosting their morale, which allows them to act in time and cast spells.
I don't see any particularly useful bonuses (for Necropolis) in other classes besides the Fire Mage, so I won't touch them.
However, the Necromancer's most useful skill is the 4th one, and if he doesn't survive, he won't only be left without experience, but he will also leave the army without reinforcements. Therefore, it is quite natural for him to be able to withstand a random attack and, if necessary, escape from the enemy. In this regard, the Assassin is a quite suitable class. This is especially useful at the beginning of the game when there is no strong army to protect the hero.

Happy New Year!