To unlock this, obtain every piece of arm gear at some point in the game. You do not need to keep them after acquiring them. There are seven pieces that are drops and will require some grinding to get.
Buy From Merchants
Wristband - Pandora
Elbow Pad - Kippo Village
Power Vambrace - Dwarf Village
Cobra Bracelet - Pandora (2nd shop)
Wolf's Band - Matango
Silver Band - Kakkara
Golem Ring - Southtown
Frost Ring - Northtown
Ivy Amulet - Mandala
Gold Bracelet - Tasnica
Shield Ring - Gold City
Lazuli Vambrace - Gold City
Vambrace - Neko (Mana Palace)
Dropped
Guardian Ring - Doom Sword (Mana Palace)
Ninja Gloves - Dark Stalker (Underground City)
*Dragon Coil - Ice Thug (Pure Land)
*Watcher Ring - Eggplant Man (Pure Land)
*Imp's Ring - Ghost (Pure Land)
Amulet Ring - Tsunami (Mana Fortress)
Faerie's Ring - Wolf Lord (Mana Fortress)
*Dragon Coil and Imp's Ring are missable if you complete the Pure Lands before acquiring them, and it's highly recommended to farm Watcher's Ring there as well.