I don't understand... how? Shouldn't the vampires kill this stack with the first attack?
On the next turn, the already damaged griffons will attack them again, and after that, the second stack will fly in. The rest will just wait... :) In reality, the battle is no more than with two stacks. Either we'll hold out, or we won't... The griffons have a damage range of 3-6, which means a lot depends on chance. It might work, or it might not... They also have high morale... So, you just need to experiment here.
But if the hero isn't leveled up, I don't think there's a chance to get the maximum score under these conditions.
