For the Dungeon faction, the entire strategy should be focused on engaging the enemy as early as possible.
The opponent will likely prepare against the Dungeon and reduce the damage from spells.
Against Orcs, the only options are Darkness and searching for minions like Berserkers, etc., so they kill each other.

Well, and you might have to resort to a not-so-pleasant tactic:
Create a group of heroes with high Chaos and Agile skills (mentoring will help) and attack them before the main hero's battle.