Wigalois and the dragon--game mechanics
"While he rode thus down the mountain, he heard great limbs crashing down and trees breaking as the dragon went along. Whatever it seized with its tail it tore down behind it; nothing could withstand its power. He thought, 'I dread to attack you--but if I am killed, what of it!' When he got a closer look at the dragon, he crossed himself and admitted in his heart that no man had ever seen another creature so horrible. He took the lance with both hands, for it was large, and his fine steed bore him toward the dragon. He lowered the lance without being seen and drove it up to his hand through the dragon's heart before it noticed him, so quickly had the man come."
So, the knight Wigalois, in Wirnt von Grafenberg's 13th century romance, one-shots a dragon. Question: what would it take to do this in D&D? We can assume Wigalois's lance is magical ("It was taken from Inner India, where there is a kind of steel which is red from the gold in it and is so hard that it cuts stone like a twig"). The dragon must have been surprised and we can assume Wigalois rolled a critical hit. He's probably a paladin, so there would be a divine smite in there.
What can we determine about Wigalois's level, strength, or feats, and the magical qualities of the lance, that would let him kill a dragon with one hit?