How to kill Black Dragons?

Sorry if we were talking about online play :) but the secret isn't in the hole itself. It's just that, like with a tank, the main thing is to get into a corner where only one dragon can reach you.

Of course. Because in offline mode, this isn't an issue. Just fly in and go for it :)
To make sure only one attacks, you need obstacles. That is, the same hole, but it's just a little easier to get into.