Keeping it in the Family

A combination from a Cataclysm strategy comment and the answer from a GM gave me the hint how to do it. For those who never raided in Cataclysm, here is what you have to do to get credit for this achieve:

1. Engage the boss by jumping down into the pit. Do not hit Onyxia. Run around waiting to get range on Nefarian.
2. When Nefarian comes down, DPS him down to around 60%. Stop DPS and don't use dots.
3. Kill Onyxia when Nefarian is at 60%.
4. Magma will rise from the floor. Get up on one of the columns and kill the add in it.
5. When you are in the column, deal about 15%-20% of his health to Nefarian, but don't kill him.
6. He will emote he's about to land. Allow him to land.
7. When he has landed, kill him.

Hope this was helpful!

This achievement is working as intended, but to get it you need to do the following. Using heroism/bloodlust is optional, but helps.

1. You will need three crackles in p1, use heroism bit before you get the third.
2. Right before the crackle hits for the third time, kill Onyxia.
3. You must dps Nefarian to below 65% while he's walking to the middle.
4. You can only dps Nefarian down by 15% during the p2. If you reach those 15% the landing begins. This is why he must be below 65% when p2 begins.
5. Have the adds finished off fast once you have burned Nefarian below 50%.
6. Kill Nefarian.

I hope Onyxia still gives epic head.