You earn this achievement by fully progressing Lucatiel of Mirrah's questline. If done properly, when you meet her at Aldia's Keep late in the game, she will give you her equipment and the achievement will pop.
You must talk to Lucatiel in several locations (always exhaust her dialogue) as you progress through the game and summon her to at least three boss battles which she must survive. Dialogue Locations
•By the No-man's Wharf bonfire
•In a tower between Mcduff's Workshop and Tower Apart bonfires in the Lost Bastille
•In the Black Gulch below the main path- drop to a ledge on the right side of the area
•Near the Lower Earthen Peak bonfire
•Aldia's Keep by the Foregarden bonfireSummon Locations
•Next to the shortcut gate in No-man's Wharf (Boss: Flexile Sentry)
•At the bottom of the lift in Sinner's Rise (Boss: Lost Sinner)
•Right outside the Smelter Demon fog door (Boss: Smelter Demon)
•Next to the second bonfire in the Black Gulch (Boss: the Rotten)
This achievement is missable but can still be gotten after defeating the final boss AS LONG AS YOU HAVE NOT TALKED TO LUCATIEL ONCE SHE HAS MOVED TO ALDIA'S KEEP. Once you talk to her there, she disappears for good and if you haven't summoned her for three boss battles, she won't give you her gear and you won't encounter her again until your next playthrough.
I got this achievement after defeating the final boss. I had never talked to her in the Black Gulch and I had never summoned her to a boss battle, so I started by talking to her in the Black Gulch to advance her to Aldia's Keep. I made sure she was there BUT I DID NOT TALK TO HER.
I then used a bonfire ascetic on the No-man's Wharf bonfire.
WARNING: this resets all enemies and loot in the area and makes the enemies in the area as difficult as they would be in NG+. Furthermore, it carries on through successive playthroughs so in this area in NG+, the enemies will be as strong as they would be in NG++. Be sure that you are willing to accept this permanent change before using a bonfire ascetic!
I dashed past everything to Lucatiel's summon sign and killed the few enemies that chased me. I then led her to the boat and sprinted to the fog door. This battle, which is equivalent to NG+, includes two smaller, super-annoying enemies that weren't in the initial playthrough battle. I did my best to stay alive for the 10 or so seconds it took Lucatiel to arrive, killed the little guys, then carefully equipped the Redeye ring so the Flexile Sentry focused on me instead of Lucatiel. Lucatiel survived the battle.
I then used a second bonfire ascetic on the No-man's Wharf bonfire and repeated the steps above to kill the Flexile Sentry a second time with Lucatiel.
Next, I used a bonfire ascetic on the Sinners' Rise bonfire. I took the lift down and killed the three big enemies and a couple of smaller guys, summoned Lucatiel, and equipped the Redeye Ring. I then sprinted to the fog door, quickly entered, and defeated the Lost Sinner with Lucatiel still alive. I did a lot of tanking (the Sinner and his two friends were focused on me), allowing Lucatiel to deal the bulk of the damage.
I then went to the Aldia's Keep bonfire and talked to Lucatiel. She gave me her gear and the achievement popped.