Find certain legends. It doesn't come out of crystals the 100% ED.
Legends that have it:
Cognition Ring, Aether Wrap, Aether Cover , Frightening Choker, Mutiny, Insolence, Frozen Shield, Arcane Blade, Entropic Jewel and Frozen Fate.
Also Seven Deadly Sins but those legends aren't worth using for any build expect for a full blown 7 deadly sins set build.
Nadroji Robe but that has Nadroji bonus despite the 100% Elemental DMG on it. But that used to be the staple legend for 100% ED before the bonus was added.
Wizard: Cognition Ring, Aether Wrap , Aether Cover , Entropic Jewel.
Warrior: Frozen Fate , Arcane Blade and Frozen Shield.
Rogue: Frightening Choker .
All Classes: Nadroji Robe , Mutiny, Insolence . Also Glinting weapon and Immortal weapon for Crystalline and Eternalized.