What is the source of the Mages’ power?

Well, I don't know how you guys play, but Titans without a penalty to their attack will defeat Paladins in two turns. I'm not an expert, but I have experience. I don't play Heroes often, but I've found myself in most situations. But in general, try it out first; I'm not just making hypotheses. A decent stack of Golems with the Golem March spell, Machine Devotion, and Luck, plus a minor artifact (+ to speed, + to luck, + to initiative, and also with light magic), will defeat Paladins while suffering minimal losses. Unlike Obsidian Golems. I personally don't take the third upgrade for Mages – it's useless and rarely helpful, except against Kshatriyas. Humans are stronger than Mages; there's nothing to compare. Razazak isn't effective; take Faiz instead. He'll reduce the resistance to magic attacks, and the units will finish them off. Or Nazir. You can fully upgrade him to level 21, and he'll have Pyrokinesis, Lucky Charm, and Empathy. Level him up correctly and find the appropriate artifacts, and then you might even see the undead horde win.