It's less the fight itself that makes him heroic in that book, and more the fact that he was present at all. By that point, he knew what was down there, and knew he didn't have any chance at stopping the snake if it went after him, but he still made the choice to go down there anyway. And then, when the person he was depending on to actually deal with the beast (as far as Harry knew at that point, Lockhart had made a career out of dealing with things like that, even if he was a git) turned on them, and left Harry on his own, he still decided to walk into the inner chamber. It's because of that that I'd say he was heroic then, and that's also incidentally the reason that the sword was able to fall on his head in the first place.
As for the monster slaying, he was a twelve year old wizard who didn't have a wand against a 1000 year old, nigh invulnerable snake, and a spellcaster that basically has a cheat enabled that lets him cast, but ensures that nothing can hit him. There's no way to pull that off without some sort of massive contrivance.
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Stand between the Silver Crystal and the Golden Sea.
"Youngsters these days just have no appreciation for the magnificence of the legendary cucumber." --Krityan Elder, Tales of Vesperia.
As for the monster slaying, he was a twelve year old wizard who didn't have a wand against a 1000 year old, nigh invulnerable snake, and a spellcaster that basically has a cheat enabled that lets him cast, but ensures that nothing can hit him. There's no way to pull that off without some sort of massive contrivance.
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Stand between the Silver Crystal and the Golden Sea.
"Youngsters these days just have no appreciation for the magnificence of the legendary cucumber." --Krityan Elder, Tales of Vesperia.