[DGDobrev]

The idea is that if you want your spells to do the job for you, you need a fast, high-initiative unit that will take minimal damage. The Black Dragon can fly from side to side and stall a slow army indefinitely till you hammer it with spells and post-effects.

As such, 1 Black Dragon is better than 1 Red Dragon. The red dragon is succeptible to enemy spells (Poison skull is awful due to 0% poison resistance, Geyser will hurt a lot due to his relatively low phys resistance), while the Black Dragon stops the enemy hero's spellcasting completely (all he can do is summon or buff) and nullifies the Evil Gremlins. Just to be on the safe side, keeping a secondary stack (even if you're not using it) of Trolls helps a lot if the AI decides to throw something unexpected at you.

You should also keep the BD as far away from enemy rangers as possible, so that they get their penalty - each ranged unit has a range, in which it inflicts full damage. When striking a target outside their range their damage is reduced for each cell outside their range.

It is about lvl 43-45 when the enemy ranger stacks grow big and numerous enough and the fear spell itself is unable to deal with them. This is when I had to swap the BD's for Black Knights for some battles. For example, 1 BD vs. 500 Shamans is winnable if the stacks are favorably set, but 1 BD vs. 500-600 shamans split in 5 stacks may give you quite some trouble and you'll be forced to keep 4 of them with fear on while you hammer the last one. In such fights it's simply much easier to swap BD's for a unit which you can ress if needed.

P.S> You can hardly imagine how many fights that single Black Dragon managed to escape with a single-digit Health value... So using the 1 BD tactics is very risky.