So, province 23 has Helheim special stages, and what they do is dealing 100% damage to all your heroes if an enemy dies. This is not a problem in normal mode. But in hard mode, it could be deadly, and I learned it the hard way. Yesterday at a stage that had 4 minions, I killed all of them at the same time, so each of them dealt 100% damage to my heroes. And that severely damaged my team. The picture below is where my heroes’ health was at as the result. They were almost at full health at the beginning. Had any of my heroes had lower health, they could have been killed by the realm bonus.
So my advice is DO NOT kill more than one enemy at the same time. Otherwise the situation could really be ugly. Good luck to all!