Either the devs nerfed mission 4 since I last played, or I hit a weird bug. I used the caster hero to quickly captured the two north-eastern towns (chain lighting ftw), and I was at an impasse because enemy AI sent stack, after stack, after stack, and wore me down with magic casts. It felt like as soon as I defeated one hero, another would spawn with an army of equal size and magic to boot. When I started up the game again, I used Cecilia with a single unit to lure away the strongest hero (bald mage lady), who got lost or stuck chasing Cecilia (?). With that mage stuck, the enemy continued to spam the other heroes at my northern-most base, but the armies were considerably weaker. From there, I built and academy and maxed out all research on increased unit stacks and bonus magic. Last boss never stood a chance.
It was nice to beat the final mission, but I feel as though I cheesed it.