Skills for an Inferno hero.

I wouldn't say that defense is just a small reduction in damage. 30% is almost a third. Regarding archdevils, I meant that before their turn, fast enemy units (like paladins, for example) can easily reach our side, and then there's no time for a gate. As for nightmares... I think it's actually more profitable to summon them through a gate because any self-respecting demon has a fast gate opening, and the initiative of nightmares often allows you to both summon reinforcements and attack immediately. Although, in general, you need to calculate the entire gate strategy; sometimes it's more profitable to just attack.
Why wouldn't Chaos be dominant? Against factions like Haven or humans, a Fireball spell with a Demon Lord targeting paladins, archangels, or dragons, etc., can be very effective.