Developing a Necromancer hero.

For defense, the most reliable option is definitely through Stone Skin. 9 defense with a single perk is a significant advantage, especially when combined with Impenetrable. Battering Ram is situational; it can be effective against Dungeon, for example. You're seriously underestimating the Sorceress. With her, you get regular stuns from skeletons (and skeleton warriors are almost always better than archers), and vampires simply prevent enemies from moving. Summoning without spells raises doubts. Even in the base summoning, fire elementals are strong. The strength of Barbarians lies in their bonus to attack and speed. With that attack bonus, you can start killing enemies with units, not just the hero. Considering knowledge and attack, Necropolis ballistae are the least useful among all factions. And a negative bonus of -5 to defense doesn't have any impact.