How to kill Black Dragons?

How to kill Black Dragons?

I don't know about everyone else, but when playing as Necropolis and using Bone Dragons, I equipped Vampire Touch and 15 Bone Dragons managed to defeat 35 Black Dragons (and the rest of the army equivalent to 35 Black Dragons, i.e., as many as you can get by having 35 Black Dragons) without losing a single one (Well, they did take some damage, but it was all restored with the next bite). My friend almost fainted (My friend is a good player). He thought I was cheating.)))

P.S. After the battle, there was no one left, not even the hero - this is like a side effect, but the main thing is that the dragons were defeated.