Weird, I just never had trouble with it, to my recollection. I know I've done plenty of moves where the guy's awareness meter went full red, he freaked out, and I would get non-detection credit. I mean, it's not to say I'm awesome at games (don't think that I am), I just didn't realize this would actually give people trouble. All I can suggest is:
1. Don't use throwing knives, as they figure you out really quickly with them.
2. Ledge-hang and brand folks whenever possible, and use those guys to do damage to packs while you hide.
3. Don't use archery a bunch in the same spot, as they'll figure you out after 3 or so shots.
Like I said, not sure what you're failing at, so I can't offer much assistance, other than the general, "don't be dumb" things. I don't know what makes this so tough for people, and that's from the perspective of someone who has never been one for sneaking games (hate Splinter Cell, not loving The Evil Within's sneak-happy approach, and a bit too gung-ho with Assassin's Creed encounters).
By the way, that guy in the video makes it WAY too complicated. If you want the bonus objective, it should be easy to get via ledge kills. However, once you decide you just want to get the thing done (or you finish the bonus kills), you shouldn't ever touch the ground. Use the bow to clear archers, and just take the rooftops to the torch bearers. Why he gets his 5 bonus kills, then bothers to brand and work his way across on ground level, I don't know.