You won’t defeat a hero with high Spell Power using a group of shamans. By learning the Darkness Nullification skill, an Orc deprives himself of something important (for example, defense), and you also need to guess which nullification skill to learn; if you make a mistake, you’ll lose. And even with all of that, the berserk effect only lasts for one turn, but that can be enough if you use it skillfully. I’ll repeat once more: you don’t need to prolong the game; tempo is the most important thing.