Demon invasion.

Hi everyone. I can’t seem to beat mission 5 at all. I’m playing on Recruit difficulty! I’m not a very strong player, but I enjoy playing Heroes. And I really want to finish this map. I’ve tried leveling up using Logistics (keen mind), Sorcery, Defense, and then in different ways: Chaos magic, luck, education... I’ve reloaded saves a million times, but every time, at some point, an enemy hero appears (a Mage with high threat, less often, when I defeat the Mage, a Necromancer and a Dwarf show up) and completely destroys me. My achievements: I captured a castle, but on my first sortie, I ran into a Necromancer and a Dwarf, they followed me all the way back to the castle, and together they beat me up. What am I doing wrong? Can anyone give me some advice? I know you’re all playing on Hero difficulty, but maybe someone can offer some suggestions.