20 black dragons cannot kill one level 1 dwarf.

Of course, this is nonsense!
Even if the dwarf has a super ability and defense of 40. Let's calculate, for example, the damage of one Black Dragon (assuming the dwarf's defense is 40):
warrior's defense: 5 (+40) = 45
Black Dragon's attack: 30
minimum damage of Black Dragon: 45
actual minimum damage of Black Dragon to warriors: 45/[1+0.05*(45-30)] = 25.7 damage units.
With a super ability, this attack will be 100% unsuccessful: 25.7 / 2 = 12.8 damage units. With skilled defense, another -30%: 9 damage. The warrior (with resilience) has 14 hit points. That is, even 2 Black Dragons will take down 1 warrior with one hit (this is with a defense of 40, a super ability, skilled defense, and resilience).
In principle, if there are 2 stacks of warriors, the hero has a lot of mana, and there is resurrection (and all the dragons are combined into 1 stack), you can revive the dwarves one by one. But mana will eventually run out, and the dragons will still eat this dwarf...

Only if this one dwarf is in the squad and there is a guardian angel.

The guardian angel is only available to knights.

Or use the rune of intangibility.

Black Dragons move earlier, and the rune only gives a 50% guarantee