I'm not even talking about Phoenix Breath or Mantis Binding. Considering they are very fast units, they will quickly reach the Djinns and Mages and destroy them in two hits. In melee combat, your ranged caster units are very weak. And if a Nature (Mage) hero casts a mass Haste, Luck, or First Strike spell, it will be the end for your ranged units. But keep in mind: a level 15 Archer hero will be even more terrifying than a Mage, given his magic resistance and devastating ranged attack.
1) Djinns have spells like Slow and Song of Peace, which will instantly cool down the attacking zeal of the Mantises and Phoenixes.
2) And if an Order Mage not only removes but also steals these spells, passing them on to his own units. Moreover, it is very effective to take not just one Order Mage, but 2-3. While one successfully removes all spells and immunities from the enemies, the second can engage in teleportation or buff his troops, and the third can cast Berserk and Hypnotize, and all this will naturally happen in one turn.
3) A level 15 Archer? I.e., we're ignoring tactics? And how will the Nature flyer interestingly reach the Order casters? If there were tactics involved, it would be a different story, with a very high chance of success, but as it is, it's all not serious. And the ranged attack is not that devastating; you rarely see more than 150-200, and besides, nothing prevents Order from defending with skeletons or illusions.
2 Lords, even with one Creature of Darkness, can clear monsters of levels 1-3 on the map (of course, with the support of Chaos Mages). And the accumulated Creatures of Darkness can be considered to be already from 7 onwards :)

Барон "Vampires"
Отдай свои прошлые сны,
В сомненьях себя обрети,
В бешеном танце душа кружится!
Не думай, что будет потом,
Здесь и сейчас мы живем,
Стань хоть на миг сам собой, слышишь!