The one time I killed a dragon was almost single-handed...
2nd level barbarian with the UA "whirling frenzy" rage variant; 22 strength while in a frenzy, +1 flaming greatsword...two attacks per round, at +6 to hit, 2d6+10 damage, +1d6 fire...minimum of 13 damage, maximum of 50 (24+20+6) on a critical.
The young black dragon was sleeping in the entrance of a cave that we were trying to get out of via the other end. The rogue tripped on him in the dark. He tried to eat the rogue, I attacked.
Two rounds, four attacks, three criticals, not a single roll on the 2d6 under 10.
The dragon was dead, and the DM had a "

" look on his face.
That character was
so overpowered it wasn't even funny...it's probably because of the stats I rolled, partly (18 str, 15 dex, 16 con, 14 int, 12 wis, 16 cha)...he went on to take leadership, get a Cleric of Sune as a cohort, and successfully grapple the head Pasha of Calimport into a dimensional toilet to the abyss...
He also helped kill the tentacled-dire-cactus-of-chaos in the Axe of the Dwarvish Lords adventure...
Then he retired with the rest of the group on a private island, because we were too powerful.