Phase Two was a little more difficult. Since you are the only one in the place the whelps all go after you. In addition it was more difficult to keep the diseases up on Onyxia and time hitting her with Death Strikes. Eventually I made it through.
Entering Phase Three, I was very low on health, got feared immediately, then my diseases dropped off.
Had I been thinking I would have Death Pacted my Ghoul which would have brought me back to half health. Retrospectively, I believe this would have bought me the time I needed to get my diseases back on and Death Strike back up to full health so I could survive her deep breaths. I didn't and with all my other cool downs blown, turned into a ghoul and died shortly after entering phase 3.
She is killable as a solo Unholy DK. A solo Blood DK will have a much easier time.
About this time a guildy logged on and asked what I was doing. I decided to cheat. Since his main was a feral druid I had him log onto his full T4 level 70 Resto Shammy.
The results speak for themselves. In fact Aup basically only healed me in Phase 2 and tossed lightning bolts the rest of the time. Gotta say, the tremor totem was nice to have as well.
So I didn't pull off the solo kill. I had fun, scored lots of vendor trash, and got to see a boss I'd never seen before.