No worms invovled. Windurst(S) must control all areas in the argoneau and sarutabaruta front, then you have to be Iron Emblem rank and farm Xicu's Rosary off of the NQ yags in Castle Oztroja(S). Once Windy(S) controls all windurst areas you can take up the campaigne OP "Plucking Wings" and it is a BCNM fight that can possibly drop this.
I recently acquired one of those, and tested it using the following gear: Aristocrat's Coat[+12%cure] & Templar Mace[+10%cure] & Asklepios[+5%cure] with 47MND gear total used Divine Seal and cast Cure V on Campaign NPC and he healed for 1756 HP. now I tried adding this earring into the formula and increased my MND gear to +50 instead of +47 and the campaign NPC healed for 1832 HP, so the difference is still minor. note: my character was WHM/NIN Hume race. its up to you to consider this as worth it or not worth it. by the way, I paid 7 million gil to get it ._.
incase you still not aware of how to get it, its random drop from the chest that appears after finishing the BCNM "Plucking Wings" which is available only when windrust in the past claims castle oztroja [s], and you have to be assigned under the Cobra Unit Mercenaries [windrust path] and have reached rank 10 [iron emblem of service] or higher. It is considered as a campaign operation. it is much more like an assault, you have to get a credit to assign for the mission [something like the ID tags for assault].
If DRG can use it, why isn't it just "All Jobs"? I mean a DRG will never at any point cast a cure, since dia/footkick-> healing breath is always better. I mean, at that point, PLD and BST could use it at the very least more than DRG.