Good point, Chrysalia. If you have CTRA, you'll be able to see the Main Tank's Target's Target. Basically, if I or another tank have Onyxia targeted, it will also display who SHE is targeting. If you happen to pop up there, she's targetting you and you should move north toward the MT.
Having 3 tanks up there is worth a try. The trick is breaking her initial fear and then laying on enough aggro to keep her. Last run, she had so much aggro on other people that my slathering of sunders, taunts, shield-slams did absolutely nothing until her targets started dying. Then she popped back to me, and we got her down to .. 14%? It was a good number
Whichever tank gets her in Phase 3, tremor totems DO help with the fears. The cavern will start to rumble as she is warming up a fear, so you can preemptively hit Zerker Rage, but there are no set intervals to her fears. A high fire resist is also helpful as she will flame-breath you, and there is also fire that comes up from the floor.
Silly as the "FIFTY DKP MINUS!" audio clip is, the crazy guy on there has a lot of the strats down. "DPS really slowly in Phase 1" ... "Phase 2, DoTs until 40%, okay stop DoTs." ... and, of course, "Positioning is important... spread out in phase 2, and stay away from the whelp caves"