Right now I'm on my 140th no-loss battle as a Mage on impossible. But that's another thing, although it is a spree.
The game shows a relative projection of how the enemy army would feel compared to your own. It is based on leadership entirely. If the enemy army has a hero, every enemy hero level over your own counts as a leadership boost to the enemy army, thus pushing it slowly up to invincible. You need not worry about that until you get to the fighting itself. Especially on impossible, you will see a ton of invincible armies in your journeys.
As for a spree - Peter Drayn, Halt, Azzak, Odin Allem and Zebat in one go, without a single loss on impossible. It is fun when you make a versatile army that can beat the crap out of the heroes
