Awakening of Evil

With the hero's attack and summoning 1 dragon, he will kill a maximum of 10 zombies and die from the retaliation.

He won't die immediately from the retaliation. Try not hitting any of them in the dungeon. You won't lose much, and when you upgrade those creatures, it will become easier. Probably...

Added 1 minute ago
And what army did you have at the start? How many scouts, beasts? For example, on a Warrior, you might have 70+ scouts, 35 beasts, etc.