Inferno tactics.

I don't disagree, but on Impossible difficulty, you have to choose between focusing on demon production (and investing all your money into it) or having a mage who flies around the map with upgraded efreeti (spending all the money on their own "education") -> which leads to synthesizing a powerful mage with minimal overhead (an unbought castle respawn accumulates in the castle itself in case might turns out to be stronger than magic, if you're unlucky with the native magic floors, and for taking those same Utopias, etc., but later on).