Also, everyone seems to be shouting their incantations, which seems just dumb.
Agreed, but they're first years. Plus, I got the impression that the dueling incantations were shouted for effect, and that Stupefy was such a large spell for them that they couldn't spare the concentration not to shout, that in fact they needed to shout in order to generate the required amount of magic (I forget what it's called, but there's a concept in some martial arts that making a sharp noise as you strike increases the strength of the blow. A similar concept could apply here).
Yes, he could have, and in some ways he should have. But would it have had the same effect as everyone saying that he defeated almost every Sunshine soldier by himself? Harry has already shown that he'll do what he needs to to boost his legend.
The term you are looking for is probably kiai.
Update: Discussion has moved on to a new thread.
The load more comments links are getting annoying (at least if you're not logged in), so it's time for a new Harry Potter and the Methods of Rationality discussion thread. We're also approaching the traditional 500-comment mark, but I think that hidden comments provide more appropriate joints to carve these threads at. So as of chapter 67, this is the place to share your thoughts about Eliezer Yudkowsky's Harry Potter fanfic.
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six. The fanfiction.net author page is the central author-controlled HPMOR clearinghouse with links to the RSS feed, pdf version, TV Tropes pages, fan art, and more, and AdeleneDawner has kept an archive of Author's Notes.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: