If I were to describe the tactic briefly: overwhelm the stacks of Priests and radiant spells and move on top of them. After that, you can breathe a sigh of relief. The rest deal physical damage, which doesn't do much damage to the Warrior units because they have high defense, and the Alliance also has lower attack than the Orcish Mages. In short, in the future, with repeated attacks, the Alliance's troops gradually melt away – gradually, only due to resurrections. Therefore, the outcome of the battle depends mainly on one thing: whether it is possible to quickly destroy the radiant spells and Priests.
In principle, the Orcs have a good chance, no matter what anyone says.
I think I've gone a bit overboard with these walls. I should take Blizzard for the hero and cast it on the radiant spells – it slows them down, lasts for 2 turns, and also deals some damage. Although walls can be useful to block the enemy's undead stack and prevent it from being resurrected. If only these walls were a little stronger... But as it is, they are destroyed by a single hit. In short, it's better to cast Blizzard on the first turn.