In Goblet Of Fire, we hear Sirius telling Harry from the fireplace
“Right — these dragons,” said Sirius, speaking very quickly now. “There’s a way, Harry. Don’t be tempted to try a Stunning Spell — dragons are strong and too powerfully magical to be knocked out by a single Stunner, you need about half a dozen wizards at a time to overcome a dragon —”
“Yeah, I know, I just saw,” said Harry.
“But you can do it alone,” said Sirius. “There is a way, and a simple spell’s all you need. Just —”
-Chapter 19, The Hungarian Horntail.
only to be interrupted by sound of footsteps (which was Ron coming) at which Harry made sure Sirius went back safe immediately. He never got a chance to speak to Sirius again before the First Task, and however he completed it successfully acting on a tip from (fake) Professor Moody.
The matter was never taken up later.
What could have been the simple spell Sirius was about to tell Harry that would help overcome a dragon all by oneself? Given that usually dozens of well trained wizards are always deployed to care for dragons?
Now Sirius is presented as a very talented wizard, at least as far as duelling is concerned, though he is not known to be so outside. There could well be some simple spell that he knew, or even he created (mind, he was one of the four who created the Marauder's Map) that would create a situation that would help a wizard overcome a dragon.
Is there any way to infer anything about this from canon?