It only drops when she's in Arrapago Reef. if you defeat her in Bhaflau Thickets she doesn't drop anything, but she is defeated before she even enters besieged.
she is assisted by 4 Lamia Exon. Two are Black mages and two are melee.
You can easily handle all four with one melee/ninja and a White mage, on the Black mage Lamia, you can substitute the White mage with a Red mage.
just have those 8 in a separate alliance while you have another take out Medusa
along with Medusa's Armlet she also drops Mercenary's dastanas
Mercenary's Dastanas RareExclusive
[Hands] All Races
DEF: 24
Ranged Accuracy +8
Besieged: Increases "Rapid Shot" activation rate
Lv. 70 WAR / RDM / PLD / DRK / BST / RNG /
SAM / DRG / BLU