Onyxia 10 man can be soloed by most classes. But it will be harder for some classes. After some experimenting i came up with a good tactic for rogues, even though they lack ranged damage. You must also be speced combat for the killing spree ability.
When you first engage onyxia do not pop any of your cooldowns. Simply dps her down untill she gets into phase 2.
Phase 2 is when she moves to one of the sides and flyes up in the air. Kill any whelps she summons and then wait untill she is at the top corner of her lair, that is important because it is the ONLY spot where you can reach her in the whole lair.
Simply run up to her and pop all your offesinve cooldowns, "jump" and then time your killing spree so that you get "teleported" up to her.
When you killing spree is done you should have done enough damage to get her down on the ground again. So after that just dps down the remaining health and collect your loot, and hopefully mount!
Good luck guys!
Ps: 14 runs here and no sight of the mount ^^