One of the best things about Dark Souls and it's community is the constant challenges that get thrown up. Onebro runs, Calamity runs, no-shield/naked runs... But Youtube user Vageta put up this interesting challenge that asks if bosses can be killed with one hit, even at a low level - and he does surprisingly well.
To ensure there's a limit to the levelling, Vageta puts some rules in place: You get two points for one-shotting a boss, and one point for two-shotting. You can use those points to level if need be, though he tends to wait until it's necessary.
He uses every trick in the book, from buffing weapons to going into full dragon form. He's also forced to use some very interesting, very hard strategies, such as combining the Red Tearstone Ring (increasing damage when you're on your last bar of health) with the Power Within pyromancy (which grants extra power while slowly depleting your health). With both active, there's only a small window in which you can get that vital hit. One shot to one-shot.
It's a lengthy series of videos, and you might want to skip ahead if you don't care about all the rules and gear, but he describes his strategies and then stays quiet for the actual boss fights, letting you both learn about the game and be entertained. Definitely an amazing playthrough.
You can catch the full playthrough, and others, over on Vageta's Youtube channel.