You require the Ruinous Shield, Orihalum, and Saints Ash for this armor. Purchase the Saints Ash from the Dragovian Village after clearing the game once (and do a game clear save, or you'll want to defenestrate yourself after your PS2). The Orihalum is earned by collecting medals and trading enough of them to the island princess north of Peregin Quay. The Ruinous Shield may be found in the air-only part of the island where the Doulmagus was killed (at the mirror puzzle).