To get this achievement by the end of Episode 2 you can buy a legendary armor set from the Ubisoft Club (need to sign-up from the game menu).
Press start and then Left on the D-Pad to get to the Main Menu and select Ubisoft Club. From here you can buy Ezio's Roman Armor set (FREE at the time of writing). The only requirement is that your character is at Level 4.
When you defeat Stentor you are awarded Nikolaos's Sword which you can engrave with the Legendary effect you obtain from Ezio's Roman Armor set (25% CRIT Chance with Hero Strike Ability).
This engraving can be done at the Blacksmith at Sami (first one available in the game).