We managed to defeat this with a PLD WHM and two Summoners:

We stack loads of DoTs on Garuda and when the feathers pop use BANE, then Sic your Egis on them while you keep hitting Garuda. Also, Fester hits Garuda for 400 each. Also with Summoners you can Sprint around the field because you don't use TP. Makes escaping death easy!

Good stuff! Be sure to tank her away from the rocks so her attacks don't hit them.