Пандора
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I don't think it's worth having two heroes from different factions in the same army – morale drops. I think it's better to take your main hero and develop them in the direction you need (the exception is a captured hero with a full upgrade of the skill I'm missing).
Otherwise, I largely agree with you. As you said, I try to develop Chaos, but in battles against an opponent from Chaos, I only use Cat's Reaction. I develop Life for healing and regeneration. I develop Order, but I often only use Teleport for quick movement around the map (if I don't have a scroll). In some cases, this is very helpful when the enemy is at the gates.
Tell me, what artifacts increase the army's defense against magic?
1. In the final battle, it's better to have two heroes. Moreover, why did you decide that they would be from different factions? Just have one focus on Chaos and the other on Life for Nature. The Guild of Magic allows you to teach them the most useful spells...
2. Order is good, but where do you get the necessary spells? What you get from leveling up is not enough. It's better to focus on Chaos and Life.
3. There are actually a lot of artifacts. I don't remember them all, of course... But I can name a few: Guardian Cloak, Resistance Cloak, Protective Cloak, Protective Ring (I think), Mind Helm (protects against Order), Crystal (also a very good item)... That's it.