How to kill Black Dragons?

In multiplayer games, I usually killed dragons with a hero who had high combat skills and a large supply of potions of immortality. In principle, with this system, the number of dragons doesn't matter, as long as you have enough money for the potions and the hero is appropriately leveled up. This may seem like an extreme and expensive method, but if a hero deals 1 hit point of damage before dying, killing 10 dragons will cost 10,000. If the army consists of archers and mages, it's better to do it this way than to lose a lot of soldiers.