Rock of Grimheim

This is the second-to-last wave. They are usually defeated by using the "Summon" skill, but I suspect that the random behavior, initiative, and morale can be very favorable to the AI and not at all favorable to the player. As for Giovanni... you could try blinding the celestial warriors with vampires. Of course, this is only possible if they don't have vampire immunity from the enemy hero. If my memory serves me correctly, the hero with darkness goes on one side.