The phrase "mighty dragon" refers to a very powerful and strong creature known as a dragon. It describes something that is extremely impressive, strong, and often seen as a symbol of strength or dominance. Full definition
Hence, when Bilbo descends into the dark cavern to confront the mighty dragon Smaug (who possesses the «treasure»), the narrator can say that Bilbo «fought the real battle in the tunnel alone, before he ever saw the vast danger that lay in wait.» (religion-online.org)
The aggregate scoreline was 2 - 1 and the Boquerones (anchovies, a club nickname) had defeated the mighty dragons of Porto. (laligablog.com)
The hero of the Street Fighter series brings his mighty dragon punch to bear in the All - Time Greatest Game Hero competition. (gamespot.com)