Killing Onyxia can provide a weekly income of 150-200g (plus any gold earned from sharding + AHing/vendoring any drops) for only 15-30 minutes' work. I've searched through the Guide Section for guides on soloing her as ret but found only guides for other classes.
DPS rotation: Crusader's Strike, Judgement (Light), Divine Storm
You should have Art of War as it provides constant heals. When ever it procs it should be used. Keep up fire resist aura for extra dmg reduction. Your seal should either be Command or Wisdom, depending on your gear/dmg. Sacred Shield can be spammed and use bubble when you're dropping too low or you want to burst Onyxia down. Also, use Divine Plea for additional mana everytime it's up.
Phase 1: Take her to the north wall with your back against it. Follow the rotation until 65% when phase 2 begins and she flies towards the roof of the lair.
Phase 2: She hovers over and starts casting Fireball. It shouldn't do too much damage with fire resist aura. Walk under her and keep dpsing. Whelps will spawn but just ignore them, Divine Storm will steadily take care of them. If there's more then 6 just take them out. She will cast a Deep Breath that wont do much and can be outhealed by an AoW FoL. Do this until 40%.
Phase 3: Take her, again, to the north wall and repeat phase 1. She will also cast a fear that will only stop your dmg for < 3 secs. Halfway through this phase you can start adding Hammer of Wrath to the rotation.
Note: I killed Onyxia as a lvl 80 paladin wearing 4/5 brutal, brutal weap and 2 80 pvp epics as well as a 70 pally with the same gear (pre-Judgement-mana nerf.) With this gear she was incredibly easy, so I assume she can be done with much worse gear. This is my first real contribution so don't judge too harshly![]()