1) Don't waste time gathering resources and building an army in the first town; instead, immediately head to the Dungeon and capture the second one;
2) I don't recommend rebuilding the second town for at least 2-3 weeks, until you've captured the nearby forts and barracks. It's easier to defend it from the advancing Inquisitors before rebuilding;
3) The first time, I leveled up my hero in air magic and primal magic, but it didn't work out, so I failed. The second time, I leveled up logistics, tactics (unit placement on the battlefield), and dark magic (it will help you a lot in the last two battles);
4) Don't be stingy with buying heroes. I had 4 by the end of the game. It's unlikely you'll be able to get more than 100% mastery with fewer;
5) In battles against large units (for example, in the third Dungeon, where archdevils guard the family heirloom), unit placement saved me. I divided the archers into 3 groups and blocked them with melee units so that the devils couldn't reach them. My losses were minimal.
In general, those who managed to pass it on the first try with a hundred mastery points are really hardcore players! For me, it's very difficult.