Been doing this along with TK for months now as an SPriest and have still yet to see it. I have horrible, horrible luck with mounts.
yeah it does drop from 10 man, I always did it 10man and I got it after few runs :P .
My sister kills Onyxia on 10 man as an Ele Shaman and it doesn't seem so hard.
I've soloed Ony10 as Elemental; its not very hard so long as you remember to keep yourself healed (the add phase hurts me more than the boss). The second time I beat her the mount dropped.
i've seen Ducksauce the dude who streams solo it as a hunter 10 man of course, he does it every now and again
I just bought it from BMAH for 250k gold even though I have 5 lvl 90s characters and could easily keep soloing it.
Then what do you know, the next day I go kill Malygos and the Azure drake drops for me.
Moral of the story is: don't be greedy, pay up and you'll get lucky with something else.
I'd have put that gold towards mimiron's head or something on the BMAH. Oh well, it's your gold. lol.
http://www.youtube.com/user/kclovesgaming <---- My Youtube Channel.
Right now I'm doing TK, Onyxia and Karazhan on a weekly basis on my Shadow Priest (my main) for mounts, no luck yet. (Been doing it for a 4-5 weeks maybe, Onyxia first time this week) Also doing Onyxia and Kara on my Warrior which is also 90. Don't think I can do TK on it though, I feel like the key to soloing Kaelthas is being ranged, but it might be doable.
Also considering trying Utgarde Pinnacle and maybe trying OS10. (i have no idea if the latter is possible)
Very quick on my Warlock, longer of my Spriest (can be tricky if you do not kill the adds quick enough) and slow on my Prot Paladin, but I just gether hundreds of Whelps for P2 to have instant Shields and Vengeance stacks.
1. Get Second Wind
2. Sit in Defensive Stance if you have mostly quest/dungeon gear, otherwise normal stance.
As for UP that's been soloable since Cata for the tanking classes, hunters and Warlocks. I'd imagine DPS could have done it back then too but might have been a gear check. Certainly a non-issue now.
OS10 3D shouldn't present an issue if you're in full heroic dungeon or better gear. )S25 3D however is a higher gear check (490?)
Took me ~80 kills before it FINALLY dropped for me on my mage on 10man about 2 months ago. Keep trying!
solo'd her every week with 5 + characters, finally dropped for me on friday at like 5am
Warlock - Main - now with GREEN FIRE!
Resto Shaman - Main Alt
Exception - Recruiting for 25man heroic progression
Stay on Ony when she lifts off, nuke her to phase 3 and quickly finish her off.
I use this strat with my main SP and my twink warlock - takes less than a minute to kill her this way. Should easily work with an ilvl of 490 and above.
I run it weekly with my army of alts which consist of:
90 Frost DK
90 Balance Druid
90 Elemental Shaman
90 Destro Lock
87 Combat Rogue
85 Prot Paladin
and have yet to see it drop. Meanwhile, it eventually showed up on our bmah at which point I began kicking myself for not saving the gold for in the event that such a thing would happen.
Owell, the run takes anywhere from 50 seconds to 9 minutes depending on the character so it's not huge hassle and it'll eventually drop.
I've been farming it for a while now on 10man, easily doable on my warlock (affliction or demonology), and ive started doing it with my dk now. So far i've only been able to do it in blood, although it takes me a ridiculous amount of time (im not that great a dk player yet...)
Disclaimer: The majority of my posts might be dripping in sarcasm. Don't take everything I say literally.