Walkthrough tips, Discussion of Campaigns [Heroes 5]

To avoid replaying the entire mission, you can try the following:

Position all your troops in one spot, summon demons from the portal NEXT to your troops, and attack the archers (using succubi, ballista, and spells from your hero).

The AI usually gets incredibly slow at this point, instead of sending its dragons to destroy your army, it waits until (presumably) the ranged towers are destroyed or the archers run out of ammunition.

This tactic allows you to win what seems like a completely hopeless battle.