This trophy is missable, if you end up talking to Lucatiel in every point of the game so that she shows up at Aldia's Keep and you did not fight 3 bosses with her and have her survive she will disappear from the game.
In order to get this trophy you need have talked to Lucatiel and exhausted all dialogue options with her whenever you see her in the game. You then need to summon her and have her survive 3 of the 4 possible boss fights.
The places you see her are:
1. No man's Wharf Bonfire.
- Allows her to be summon for the Versatile Sentry Boss. Her signal will be near the door where you pulled the level to get a shortcut passing through the level.
2. Lost Bastille (McDuff Bonfire) - Head straight up the stairs, beat the enemies and she is in the tower.
- This will allow you to summon for for the Lost Sinner (Sinner's Rise) boss battle. Her signal will be in the basement after you take the elevator down to the holding cell area.
3. Lower Earthen Keep Bonfire.
- This will allow you to summon her for the Smelter Demon (Iron Keep) boss fight. Her summon sign will be right before the boss door.
4. Black Gulch (Mouth Bonfire) - Under that bonfire, you have to drop down to a small exposed ledge right outside the bonfire.
- This will allow you to summon her for the The Rotten (Black Gulch) boss, her signal will be at the Hidden Chamber bonfire in the Black Gulch
5. Aldia's Keep (Foregarden) - This is where she gives you the equipment, she will only show up here if you've talked to her in every other location.
You do not have to meet the NPCs or fight the bosses in the order they were listed. You can also summon them for the same boss in the same playthrough by using Bonfire Ascetics as confirmed by players.
If Lucatiel dies you can simply choose to die and redo the boss battle resummoning him or quit the game and reload it and summon him again. If you've already completed a boss battle I would recommend just trying to go for this trophy in NG+ but you ca use the Bonfire Ascetic item on the bonfire closest to the boss to respawn that boss and summon Lucatiel for that battle again. I do not advise doing that because the bosses will be harder making it harder for Benhart to survive, so only do that if you are extremely overpowered and can kill the boss quickly or have another strong human phantom to help you kill the boss before Lucatiel is killed.