In order to execute this properly, it is imperative to note when the phases end. Phase 1 begins when you engage Onyxia and ends as soon as Onyxia dies. Phase 2 begins when Onyxia dies and ends when Nefarian has taken 15% more damage, or when all three adds are dead on the pillars, whichever happens first.
In order to complete this achievement, you have to push Nefarian to three crackles in phase 1 and get him below 65%. Because each crackle boosts Onyxia's electrical level, you must coordinate to have Onyxia nearly dead and then kill her as the third crackle is going off while continuing to DPS Nefarian down. In phase 2, then kill two of the adds but leave the third one alive so DPS can focus on Nefarian -- he needs to be pushed through two crackles while still in the air in order to complete the achievement successfully.
- Patch 4.0.3a (2010-11-23): Added.