One Man vs Onyxia - if you hear that title you probably think "fake" or "exploit", however it is neither. Onyxia CAN be legitimately one-manned as this movie will hopefully prove to you.
You will not see her dying here. For that I would have needed tremendous amount of luck with the pathing of the main Warder in the entrance tunnel - which I hadn't in the many, many tries - but you will see everything up until mid-phase 2 which is all the challenge this encounter has to offer.
That "main" Warder is a "flaw", however you only find that out if you try to solo her - that is: if you need more than 30 minutes to get her to phase 3 - so no one ever really noticed it.
In Patch 1.3 Blizzard made the Warders respawn upon engaging Onyxia to discourage people from hiding in the tunnel from her. However that "main" Warder who patrols through the entire entrance tunnel does not respawn for one obvious reason: His patrol path leads him almost to Onyxia herself and he would very likely add in every fight.
Now the problem is that this warder still has his default respawn timer of around 30 minutes after which he will spawn again even though you are engaged with Onyxia.
Edits The FRIEND issue
it is possible to have a friend pick up the Warder but then it's not a real solo anymore since the fireball damage will start to spread on 2 or more targets as well as the whelps eventually.
The choice of gear you can use here is very limited. You need 350+ fire resistance or you take too many Flame Breaths and it becomes unhealable. I used the best in-slot equipment for everything except Head/Neck and I doubt those two will make a difference. (Chest/Gloves/Legs/Boots/1xRing MUST be full fr-gear or you can't reach the 350)
P2 and P3
As I stated in the movie, P2 is not a problem since you can dps Onyxia with swipe and in between even in cat form a bit. Yes, it would take VERY long (probably ~2 hours for P2) but since there is no mana problem and the incoming dmg is very low you can do P2 forever.
P3 is exactly the same as P1 with the only difference that there is the fear (3s duration, with 1.5s casttime) and the Lava Crack Eruptions. The fear is no issue since I mitigate 80% of Onyxia's dmg via fire resistance and armor, furthermore I have skilled 15% fear resist and if there should arise any problems you have 1.5s time to throw in a Potion/NS Healing Touch and reshift. The Lava Crack Eruptions are resisted more than 95% of the time since these are some kind of lvl 60 based environmental spells and they only occur during fear.