That's a hasty judgment. I remember fighting against Haggeesh Letos, and throughout the entire battle, Phoenixes with Shield and Vampirism, and Wolf Riders with Subjugation did all the work. It was a full-fledged Darkness build – he gathered troops from Treasure Rooms and Utopias (the map was large and rich).
When we played the rematch, the opponent took Vaishan against Haggeesh and went for a Spell Power build. At level 29, he only had 22 Spell Power due to my Magic Resistance perk. And we also need to subtract 25% from that due to Chaos Weakness... In general, the main damage was dealt by the troops, not magic.
Against the Orc, the Spell Power Warlock is really effective because the attack or spell removes the Shield of Rage, and the chains then hit the "bare" stack.
300+ Minotaurs took down 4 Cyclops with the first hit, took some damage in return, after which there were 181 left, and with the second hit, they eliminated 41. 400+ Warmongers also took down 4 Matriarchs with a single shot... However, it was impossible to win that battle with just an army. If it were Darkness, I would have lost. Or, if I had taken Summoning as the second school – there are some fun spells there...
Bow down and hear our cry,
Our earthly rulers falter,
Our people drift and die,
The walls of gold entomb us,
The swords of scorn divide,
Take not thy thunder from us,
Take away our pride.
G.K. Chesterton: English Hymnal