Summoner Belinda:
To get the fifth Resonance Amulet, you must slay the Archon of Halisha. Find that devil in the Imperial Tomb or at the Wall of Argos.
If you go to the Imperial Tomb, you'll find him in the Four Sepulchers. At the Shrine of Loyalty, the Archon will appear after you destroy many of his minions. His death is essential if you are to get the Resonance Amulet. Good luck!