I saw this thread and wanted to give it a try myself.
I went in (I'm fresh 80, turned in the night before I did this) Killed a few of the dragons in the begining and got to Onyxia!
Phase 1 = Seal of Wisdom, Holy Shield, HoR, Judge of Light, Fire Aura up at all times. I also kept Sared Shield and Divine Plea up consistantly. It took me a bit but I got her to fly finally.
Phase 2 = Seal of Light, Holy Shield, HoR, Judge of Light, Fire Aura up at all times. I needed to do SoL and JoL because the whelps did add a lot of dmg that I needed to keep myself up. Having Blessing of Sanc up kept my mana considering the whelps never were able to hit me. I did however have to use my LoH here, I probably could have avoided it but choose to be safe. The hardest part is keeping Onyxia "In front" of you due to her moving around a lot.
Phase 3 = Seal of Wisdow, Holy Shield, HoR, Judge of Light, Fire Aura up at all times. Kept Sared Shield and Divine Plea going as well. The final phase was fun, same as phase 1, but just the knowing that I was going to do it got my heart beating!!
I ended up downing her in 41 minutes total (Stam food timer) From opening the front door of Ony's Lair to Looting her. It was fun talking in Guild chat a little while doing this, I was counting down her HP occasionally and there didn't realize I was actually SOLOing her. Got done and there like WOAH!!!!
Kinda cool, I LOVE my class no other class can compare!
Here's a short youtube of my killshot/acheivment just so ppl didn't question me.
http://www.youtube.com/watch?v=ZJdIUoC1pUw
Here's my armoury also
http://www.wowarmory.com/character-sheet.xml?r=Spirestone&n=Poneress