Worshipper – “Betrayal” [2nd campaign, 1st mission]

Nah... This mission can be completed normally on the second-to-last difficulty level (Veteran, I think) - I've done it. Moreover, I deliberately attacked all neutral units that I could defeat without losses (or with minimal losses) to level up the hero (I generally try to max out the hero in each mission). And the main part of the troops is recruited from the Inferno neutrals that join you (I think all of them join, except for the demonesses) and by progressing through the dungeon in the buildings. Then, it's desirable to capture the fortress (I think that's what it's called) to upgrade the un-upgraded units. And that's it - victory is in the bag.