CREDIT TO "vSully" on TrueAchievements for the following guide, it is not mine, but it is accurate, unlike the other guide on this site:
You earn this achievement by fully progressing Benhart of Jugo's questline. If done properly, when you meet him in the Memory of Orro late in the game, he will give you his equipment and the achievement will pop.
You must talk to Benhart in several locations (always exhaust his dialogue) as you progress through the game and summon him to at least three boss battles which he must survive.
Dialogue Locations
•Between Majula and the Shaded Woods. Talk to him before and after you unpetrify Rosabeth.
•Next to the Central Castle Drangleic Bonfire
•Near the Pursuer's bird nest in the Forest of Fallen Giants
•Near where you start in the Memory of Orro
Summon Locations
•In one of the tents near the Royal Army Campsite bonfire (Boss: Prowling Magus)
•Right outside the Looking Glass Knight fog door (Boss: Looking Glass Knight)
•Near where you start in the Memory of Jeigh (Boss: Giant Lord)
•Right ouside the fog door to the final boss (Boss: Throne Defender/Throne Watcher and Nashandra)
GOOD NEWS! As far as I can tell. this achievement is not missable and can be gotten after defeating the final boss but before moving on to NG+.
I had talked to Benhart at each location as I played through the game, but I had never summoned him. After defeating the final boss I was able to go back and get this achievement rather than waiting to fulfill the requirements in NG+. Here's what I did:
I started by using a bonfire ascetic on the Royal Army Campsite bonfire in Brightstone Cove Tseldora.
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!
Benhart's summon sign is inside one of the other tents near the bonfire. I killed all the enemies in the area, then killed the rest of the enemies along the path to the Prowling Magus battle. I then went back and summoned Benhart, then defeated the Prowling Magus with him still alive.
Next, I used a bonfire ascetic on the Central Castle Drangleic bonfire in Drangleic Castle. Benhart's summon sigh appears just before the fog door to the Looking Glass Knight. I summoned Benhart and defeated the Looking Glass Knight with him still alive.
Next, I used a bonfire ascetic on the Place Unbeknownst bonfire in the Forest of Fallen Giants. Just beyond this bonfire is the tree that you interact with to enter the Memory of Jeigh. Benhart's summon sign is in the memory, at the dead end straight ahead from where you start at the bottom of some steps. I summoned Benhart, entered the fog wall, sprinted past the two giants to trigger the huge rolling head, dodged it, then ran back and helped Benhart kill the lone surviving giant. We then killed the Giant Lord with Benhart surviving.
Finally, I went into the Memory of Orro and talked to Benhart, at which point he gave me his equipment and the achievement popped. Before I went back and used bonfire ascetics so I could summon Benhart to the boss battles, he would talk to me in the Memory of Orr but he would not give me his equipment.
Since he never seems to disappear from the Memory of Orro and you can go back and fulfill the requirements at any time before moving on to NG+, this achievement is technically not missable.
NOTE: If you're having trouble keeping Bernhart alive during boss battles, buy the Redeye Ring from Sweet Shalquoir (the cat) in Majula. With this ring equipped, enemies focus almost entirely on you and not on your allies.