Onyxia Copied and Pasted - THANKS Guys! :)

Phase 1

Special Attacks:
Fire Breath: Cone shaped fire AE that hits everything in front of her for 3000-4000 damage. Solution: Always keep her pointed north, away from the groups on the sides.
Knock back: AE knock back that throws you really far, likely well into the eggs if your back is to them. Solution: Correctly position yourselves at her sides as far away as possible while still being in melee range, keep your back away from the eggs.
Tail Whip: AE 2-3 second stun that hits everyone behind her and does 200-300 damage. Solution: Never stand behind an angry dragon.
Positioning:
Odd numbered groups run to the NW corner:
West side/odd number group positions:W = Whelp GroupR+C = Ranged and CastersM = MeleeMT = Main Tank
Even numbered groups run to the NE corner:
East side/even number group positions:W = Whelp GroupR+C = Ranged and CastersM = MeleeMT = Main Tank
Big screen shot: overall positioning.
The south is a death zone.
Plan:
MT rushes in a good 20-30 feet ahead of everyone to make sure he gets initial agro. No one is to attack Onyxia until the MT is in position and has been in position building agro for at least 60 seconds.
Whelps that are still spawned from the last wipe are pulled off the MT with light AE damage (ie: Consecration) and dealt with quickly. There are usually only a few whelps on each side so this is never a big deal.
Groups 7 and 8 deal with whelps on their sides in the marked area. These 2 groups should be doing nothing but whelp duty throughout all phases. During phase 1 whelps will spawn at ~30 second intervals and come ~10 at a time. One group on each side can handle them without any problem. If someone is poorly positioned and gets knocked into the eggs, they are about to train 20-30 whelps on their side. Groups on that side can stop attacking Onyxia and kill the large wave of whelps, but the run is likely over since dealing with a whelp zergrush damages our long term survivability significantly.
MT slowly moves into position at the north center of the lair. The MT's exact position is very important due to the knock back. He needs to be 10-15' away from the north wall. When positioned correctly the knock back should bounce the MT against the north wall and have him land very close to his original position.
Onyxia should always be facing north.
Once the MT has built 60+ seconds of agro, everyone can attack. Mind your DPS, hunters FD as often as you can, rogues feint when it's up, mages don't over nuke, and so on. If the MT loses agro, she will turn to one of the sides and kill everyone with a Fire Breath.
At 80% life everyone but the MT stops attacking for 30 seconds for agro management. A heal rotation can keep the MT alive forever during this phase so there is no rush to kill the dragon.
Keep attacking slow and steady until 70% life when we take another 30 second break.
Drink a fire absorption potion during the break.
At 65% she will walk from the north to the south and take flight. Phase 2 begins.
Ramblings:
Agro management and MT positioning are the most important parts of this phase. This phase is remarkably easy compared to the next two due to its straightforward "MT tank with a heal rotation, everyone else kill slow and steady" strategy. There should be 0 deaths and no whelps alive when phase 2 starts. There shouldn't be any need to drink any potions (except the fire prot.) or use anything with a noticeable cool down during this.

Phase 2
Special Attacks:
Engulfing Flames: She spits fire at random people. When the fire hits you, you're on engulfed running around confused and will take around 1500-2000 damage over 5 seconds. Even worse, it will spread to anyone near you. And, of course, it is not dispelable by anyone. Solution: Do not cluster together. Stay a good 5-10' away from the person closest to you.
Deep Breath: She will emote "Onyxia begins to take a deep breath" and CQOnyxia will warn you a deep breath is coming in <5 d =" The"> = Fiery death.Solutions: If you aren't bunched together and spread nicely apart, deep breath likely won't ever even happen. She will only do deep breath when everyone is bunched together and she will intelligently target the largest bunch of people. However, you need to be prepared if it's coming, run to the wall, as far as you can go against the wall, away from where the breath is going to go. We have people on Ventrilo giving position updates constantly so we always are ready to move out of the way if a deep breath is coming.Currently the entrances to the eggs are safe from deep breath attacks no matter where she is. This is changing in the very near future according to patch notes.
Positioning:
West side positions:X = Players
East side positions:X = Players
As you can see, everyone is well spaced apart so when someone is on fire it shouldn't spread to anyone else. Also note you are pretty much where you were during phase 1.
Onyxia will be flying around in the lair so people doing ranged damage need to be keeping in range. Your healers know you are going to be moving around and will stick with you, but try to stay as close to your side as possible. It's okay to do ranged damage from most of the lair as long as you aren't clustered together and get to the wall immediately if she is going to deep breath.
Plan:
Right as she goes into the air the first wave of whelps is like 15-20 so everyone needs to help with whelps immediately. After the first wave is dead groups 7 and 8 are still solely focused on keeping them under control. If they get out of control everyone needs to help deal with them and go right back to attacking Onyxia once they're dead.
Everyone moves into position and spreads out. Warriors are now taunting whelps off of casters since they are worthless at ranged. Paladins are AEing whelps and healing. Groups 7/8 are ignoring Onyxia, everyone else is doing ranged damage to her.
Healers need to be watching for people burning (especially Priests). Use first aid.
Ranged damage is the most important thing. The goal is to get her back on the ground as quickly as possible. Do every DoT, curse, damage spell you have. Zap wands. Currently melee people can still use their special abilities when she is in the air if they are positioned directly under her. This will be changing in the next patch. Remember this phase is long and you will need to recast curses and DoTs as they wear off.
At around 45% the MT starts building rage on whelps to pull Onyxia back onto him when she lands. At this point ranged DPS should be unloading, the whelps should be under control, and the MT should have rage.
40% she is going to land any second (we've seen her land as late as 35% but it's almost always at 39-40%), MT moves under Onyxia ready to get agro and everyone stops attacking the second she is on the ground. Phase 3 begins.
Seconds before she lands we move away from the egg entrances to avoid being feared into them.
Ramblings:
The whole key to this phase is positioning. If you are spread out enough Onyxia will only hit a couple people at a time with Engulfing Flames and she will likely never take a deep breath. Even on our best executions of phase 2 she has taken a deep breath. This isn't a big deal at all as long as everyone is ready to react and knows exactly where to go. A hunter is designated on Ventrilo to give us updates on Onyxia's position. The only hard part about this phase is the last 5% of life right before she is coming back to the ground. Then the transition into phase 3 is the hardest part of the entire encounter.

Phase 3
Special Attacks:
Everything from Phase 1.
The cracks in the floor shoot lava every 20-30 seconds fearing everyone near the crack which usually sends them running through other lava cracks for a good 1500 damage. The fear isn't dispelable and fear ward doesn't work. Solution: Stay off and away from the cracks. Hug the wall. When you get feared, immediately get back into position. Mithril Insignia works for anti-fear but switching stances to break fear is dubious at best for the MT.If you are moving forward and backwards rapidly while meleeing you will not run around when you're feared, you'll stand in one place feared. Anyone who can should be rapidly moving forward/backward to avoid being feared through lava.
Positioning:
R = Ranged/CastersM = Melee
The positioning is the same as phase 1 but much tighter due to the exploding lava cracks. Remember, it's now okay to bunch together again so cluster up and stay off the cracks. Melee stay as far away as you can off the cracks. Ranged people hug the wall away from the cracks.
Be careful not to get too far in front of her along the wall. The MT will slowly be moving her north, don't go too far north and get hit by the fire breath. Keep your back away from the eggs incase you are knocked back.
Whelp groups get a break because there are very few whelp spawns as long as no one gets knocked into the eggs. They need to be positioned further away from the eggs so they aren't feared into them. Near the leftmost/rightmost R spots on the above map.
"Plan":
Big dragon lands, MT is waiting under her with rage, slaps her in the face, and questions her mother's sexual orientation. Nobody else is attacking and the MT has agro.
MT moves to the phase 1 position but mindful of cracks in the floor now.
It's likely that the MT will get feared and lose agro before he's in position. Assuming the MT has enough agro she will chase him around while he's feared. If she doesn't, #2 on her agro list is promised a swift death. If she fire breaths a bunched up side while the MT doesn't have agro the run is over.
The MT is going to lose agro more than once because he gets feared, teleported, knocked around, and downright violated by her special attacks. Stop attacking, get ready to move to avoid fire breaths/eggs, and move back into position for attacks when she is back on the MT pointed north.
Beyond the floor exploding with lava, being feared into the lava, the MT losing agro or changing positioning often, and everyone being very low on survivability, this is exactly like phase 1. Mind your DPS and position. Deal with the whelps. Let the MT tank and keep him alive at all costs. If the MT dies even at 5% the raid is over.
Anything that can do damage should be focused on balancing DPS and surviving. Anyone that can heal should be healing the MT. If your priests say the MT is under control backup healers should be keeping DPS alive.
At around 5% her damage output will go up noticeably (by 25-30% I'd guess) and pretty much everyone in the raid who can heal should be spamming their fastest ones on the MT. Like I said, if the MT dies at any point the run is almost certainly over.
Don't die.
Reduce the dragon's life to 0% calmly and steadily. Keep things under control.
Ramblings:
Total chaos. Stick to the plan, stay calm, and be at the top of your game. Your MT has a lot of learning to do during this phase. He needs to be ready to deal with fears, agro, positioning, knock back, teleports, everything. The MT makes or breaks this phase.
The most important thing everyone on the raid has to know is not to get overconfident when she is really, really low on HP. We had a run where we had her down to 5% and had the people alive to kill her, but in a blink of an eye here's what happened: 2 extremely experienced people got knocked into the eggs (it happens, this phase is very chaotic), 30 whelps come out and start one shotting ranged DPS dealers along their side, heals from that side are focused on the whelp killers instead of the MT, the MT dies due to lack of healing, Onyxia and her 30 whelp minions go on a killing spree and we wipe at 1% hp. The entire breakdown happened extremely quickly as you can watch in the video below.

Specific Techniques that work and don't work:

Heal rotations. Obvious big one. Only really effective during phase 1 in my opinion. We usually have 3 priests in rotation healing to 25-35% mana before swapping out. The MT should be able to literally solo Onyxia down to 60% if he wants to since we can keep him alive forever via rotation. During phase 2 people really aren't taking that much damage. Engulfed people are easily healed with first aid 80% of the time. The other times not much more than a flash heal or, very rarely, PW:S is needed. I, as a priest, usually end phase 2 with nearly full mana. We've got into heal rotations for brief periods during phase 3 but with the fears and the amount of damage the MT is taking (especially when she gets really low on HP) it would be impossible to not keep him alive without everyone healing as fast as they can.
Offtanking. Never really did it. If our MT died the run was over for us. A few times a second warrior successfully obtained agro but he never would last very long. We preferred simply never letting our awesome MT die.
Carefully popping eggs for a more controlled whelp killing. Tried it. Didn't work. They respawn too fast and your whelp teams should be able to deal with the waves as long as no one is knocked into the eggs.
Turning Onyxia south during phase 1 if/when the tank gets knocked far to one side. Tried it in our very very first runs with little success. Once your MT knows how to position himself so the knock back isn't a big deal, Onyxia should never ever face anywhere but north during phase 1 and the majority of phase 3.
Hunter AE frost slowing traps at whelp entrances. I would remind the hunters to lay a trap if they could at the very start of phase 3. One hunter accomplished this once. They simply get into combat too quickly after FD to lay a trap. I guess it would be worthwhile if hunters could consistently get traps down at key moments.
Battle resses! Obviously. Druids would battle res priests usually towards the end of phase 2. Priests would come alive and immediately res someone useful near them before they are put into combat. Sometimes they got someone ressed, sometimes they died instantly upon being battle ressed, sometimes they were put into combat immediately and never got a chance to res. Also, the more you wipe for a night the less effective this gets due to res timers on Druid resses (being patched though).
Double soulstones. Ideally everyone would begin phase 1 with around 7-10 minutes left on their soulstone. That way if they died during phase 2 or phase 3 they could reincarnate and get another stone a few minutes later thanks to cool down. Honestly, we never really used this to it's full potential. And I never even used my first soulstone on the run where we killed her. Reality is, if you do the phases correctly you shouldn't have your raid relying on battle resses or soulstones in the slightest. These people shouldn't be dying in the first place (but it happens).
Blasted Lands Quests. Get your MT +50 STA things from Blasted. Healing a MT with 7500 hp is pretty damn easy.
Cozy fire is essential at all times in all positions. Fire! Her one weakness! We should have known!
Your chances of success go up exponentially with every ninja and/or pirate at the run. Also, when your MT dies the only way to beat her is by having a paladin pirate start tanking and furiously smash his win button while dancing.
The Warders dropped 1 blue the entire time. It was a nice mace. We killed around 150 Warders.

Videos/Screen shots:

Onyxia Kill from JOINKTHEWIZARDLVL700 (best video) 3-17-05Onyxia kill from Kilranin the Paladin 3-17-05 he gets ported under the world at the endWipe at 1% from Kilranin 3-17-05Wipe at 28% from Joink the Mage and Varlock the Priest 3-16-05Arkaine tanking Onyxia while turned into a pirate dancing
The kill raid groupThe body shotThe loot (grats to Licid, Rapportus, and Gilliam) A rather awkward encounterSo easy to heal the MTAnother dead shotDance party in IF afterwards + video

Credits

Information was compiled with the help of , , , on Kel'Thuzad Alliance over 4 nights of raiding. We all rushed into our first run with only the plan of "Odds go NW, evens go NE, let's see what happens" and over the next 4 days we compiled the above strategy that works using only knowledge we gained after each of our 45+ attempts.There are so many more players who put serious amounts of time into our Onyxia journey that weren't present for the kill. We couldn't have done it without everyone who participated, even you lame east coast people who claim to have jobs.This guide was made by Hocken of Contact(specific questions will likely not be answered, please use the forums for that)The 1% wipe and the kill were supervised by several GMs and every strategy here has been verified legitimate.
"... the biggest challenge the fight poses is that individual player performance is tremendously important. In many other 40-man fights, if you have five idiots, those five idiots will die and the other 35 can finish the job. With Onyxia, any one player can pretty much sabotage the entire raid, rather than simply getting himself or herself killed. I'd say the key to our success was trying to get as many of the same people for each attempt, of which there were many. We've been wiping to the bitch all the past week, pretty much, including one heartbreaking wipe at 3%. But by bringing back as many of the same people as possible, we benefited from our experience in all those wipes instead of having new people coming in and making the mistakes we'd all learned not to make four wipes prior.
The fight is more about executing on an individual level than any grand master plan. To put it in simplest terms, stage 1 is all about agro management, stage 2 is all about positioning, and stage 3 is just chaotic as all hell.
It's just a matter of getting a group of people together and staying motivated through the required wipes needed to learn the encounter."-- Gurgthock Mal'Ganis

IT'S JUST A DRAGON.