Well, it sounds quite convincing overall. You're making a good point. This approach could be used to level up for a long game, as it requires a lot of levels to put all of this together. But no matter what you say, dark magic can be more useful at this stage of the game. Some kind of debuff can easily compensate for the lack of defense of a demon. The Fire Lord will be especially useful against tough opponents (humans and dwarves). Even though 3 dwarven creatures have immunity to fire, the Fire Lord still affects them, as far as I know. And reducing the stats of Yargle and/or dragons is very, very cool... Humans, on the other hand, don't have such well-protected creatures, but their hero adds a lot.
P.S. In general, I think it's quite logical to increase the demon's defense, as the chance is not small. Learning the Stone Skin perk there... And if you're lucky enough to also learn Darkness and Weakness from there, then demons will also become tough.
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