Well, it goes without saying that there’s no point in taking Lethosa against Necromancers. As for the expediency of choosing Erinn:
1) At the beginning of the game, there are either no witches in the army, or there are very few. They mainly serve to support with spells. They are unlikely to be called great archers, and it is unlikely that anyone will enhance them by taking the Archery perk, for example.
2) In a prolonged game, the effectiveness of destruction magic sharply decreases. In order for the witches to shoot, the hero must cast some kind of MP spell. Well, the hero will destroy 10-20% of the enemy stack (maybe even less) with a spell, and the witches will kill about the same with their shot (if they shoot and if they haven’t been killed yet), and will this really help? It is much more effective to cast Berserk / Mass Haste, Stone Skin, Dodge / summon a Phantom or Phoenix, IMHO.
Against the same Necromancers, I would take Iranna. Both the specialization in main units and the ability to develop any school of magic, and Education as a basic skill, which will solve the problems with mana and allow you to quickly get a mentor. And this, in turn, will make it possible to constantly send upgraded roaches with a few stalkers to the enemy, which will significantly reduce the Necromancer's troops.