Quick and Easy (Pay) way to get this is to buy a gear pack from the in-game Helix store from the menus.
*** Spoiler - click to reveal ***Not sure what level you need to be to wear these, but here's the info from the store
Armor only (Gear tab)
500 Helix creditsSpartan Renegade Armor
Nemean Lion Pelt
Odysseus Armor
Celestial Suit
Athena's Blessing
Wild Boar Armor
Abstergo Suit
Shark Armor
750 Helix creditsPegasos Armor
Hades's Champion
Armor of Dionysos
Sphinx Mane
Armor + Mount + Weapon (Packs tab)
1000 Helix creditsSpartan Renegade Pack
Nemean Lion Pack
Odysseus Pack
Celestial Pack
Athena Pack
Wild Boar Pack
Abstergo Pack
Shark Pack
1500 Helix creditsPegasos Mythical Pack
Underworld Mythical Pack
Dionysos Pack
Sphinx Pack
OR
Not-so-Quick (Free) 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.
GUARANTEED ARMOR
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.
GUARANTEED WEAPONS
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:
*** Spoiler - click to reveal ***Snake (Eyes of Kosmos cultist drops)
Agamemnon (The Silver Vein cultist drops)
Athenian War Hero (Delian League cultist drops)
Spartan War Hero (Peloponnesian League cultist drops)
Pirate (Gods of the Aegean Sea cultist drops)
Immortal (Worshippers of the Bloodline cultist drops)
Achilles (Heroes of the Cult cultist drops for Alexios)
Amazon (Heroes of the Cult cultist drops for Kassandra) thanks ForbiddenJobu
Pilgrim (sacred temple legendary chests)
Greek Heroes (Mercenary drops)
Arena Fighter (Arena contender drops)
Artemis (Daughters of Artemis animal hunt rewards)
Demigod (finish the story: defeat all cultists, confront Deimos)
Abomination (Hard Mode Mythical Creature drops)
Legacy of the First Blade DLC
Persian Elite (Order of Hunters cultist drops) Episode 1 Hunted
Master Assassin (Order of Dominion cultist drops) Episode 3 Bloodline
Fate of Atlantis DLC
First Civilization (various quests) Episode 1 Fields of Elysium
Pegasos (store)
Spartan Renegade (store)
Nemean Lion (store)
Hades (store)
Odysseus (store)
Celestial Suit (store)
Dionysos (store)
Athena (store)
Wild Boar (store)
Shark (store)
Sphinx Mane (store)
Abstergo Elite (store)
store = Menu store, not in-game blacksmith shops
Equip your Legendary weapon of choice. Then equip all 5 armor slots as Legendary. Aaaaand, achievement.