Heroes 6 - Discussion

Hello! I'm looking for some help. I prefer to play our favorite game in the following way: I complete all the maps and only then do I start the campaigns, saving them for last. I've been playing Heroes for over 10 years. I played the second, third, and fifth installments, and I completed everything in all of them. BUT in the sixth game, I get completely crushed! It's so frustrating... I play on "normal" difficulty. I do everything, and it seems like I'm doing it correctly. I develop, build, accumulate an army, destroy everything around me, visit all the obelisks, and level up as much as I can. Then the AI shows up with level 14 (and I'm only level 6, for example) and just crushes me! But I can't bring myself to set the difficulty to "easy." After all, in the fifth game, I heroically fought on veteran difficulty...! What tactics should I use, friends? What strategy should I follow? What should I do?!