Quick and Easy way to get this is to buy a gear pack from the in-game store.
Not-so-Quick way is to earn these in the game. After Chapter 4 starts, you can hunt cultists. Since you need all cultists erased anyway, just go for this when you're able.
You get a full set of Legendary armor every time you kill all 6 cultists in the same branch of the Cult. (Five armor from cultists, plus weapon from Sage)
Some mercenaries also drop Legendary armor.
You are given a Legendary sword at the end of Chapter 2.
Some cultists (Sages) also drop their own Legendary weapon.
Some mercenaries and arena fighters also drop Legendary weapons.
You can see what loot the Cultists and Mercenaries drop by looking at them on their Menu pages. Hover over known/identified characters to see what they have.
Known Legendary armor sets include:
Equip your Legendary weapon of choice. Then equip all 5 armor slots as Legendary. Aaaaand, achievement.