Now, Bard was the only PC there, but he played it smart. He sent all the NPC archers to deal with the dragon....to face its breath weapon, to get battered by its tail, etc. Then, when he saw that the dragon was weakening, he fired his one magic arrow...and that with a circumstance bonus to hit provided by a thrush (read: DM PC) telling him where to aim. A whole bunch of NPC mooks die, and Bard becomes "The Dragonslayer".
Canny action on the part of the player, if you ask me. Perhaps a bit of DM favouritism, too. I imagine that most players could beat most threats if they could just get the NPCs to whittle them down for a few minutes first. And then they, too, could take the credit for the final, killing blow. "Really, guys, your arrows were doing nothing."