This is how I soloed Onyxia as level 80 affliction warlock.
I didn't want to change build so I decided to challenge myself and have a try with a completely oriented raiding talent build.
After a couple of tries I managed to solo kill Onyxia, it's not an easy fight but definitely doable with decent gear.

After all, drain tanking is still good in WotLK...

http://www.wow-europe.com/en/info/basics/talents/warlock/talents.html?tal=225022200002351005250033135100000000000000000000000000045012005000000000000000000 .