I played on normal difficulty, and everything went as described. My army consisted of: 100 pikemen, 30 sorceresses, and 10 liches. Their army had around 80 succubi, up to 28 summoners, and so on – a large number of units. My advice is to start the mission from the beginning. You might also run out of mana. My hero has 630 mana, and the hired magic master has 350.
I summon 22 dark elementals – that’s a powerful force. And I have 18 of them for each of my two heroes. You just need to eliminate them in 4 turns. Also, constantly control the gates. As long as you’re in the fortress, you’re invincible. Although I have almost a full magic set, and my lightning strike deals 1400 damage, even a purchased hero, leveled up in magic (and I buy him at level 19), shows very good results.