DARK SOULS II: Scholar of the First Sin

This achievement needs some planning to make it as easy as possible so try to do things in the order I list them in order to make life easier on yourself. For more information on specific bosses/locations my walkthrough is here.

IMPORTANT: Her locations have changed for Scholar of the First Sin, so be sure not to miss her. You can talk to her in ANY order, but you need to speak to her in No Mans Wharf and in the hidden tunnel at the start of Black Gulch to make her available for boss fights. Her last spot is ALWAYS Aldia's Keep and she needs to have survived three fights BEFORE you speak to her here. This has to be done in ONE playthrough.

In No Mans Wharf (past Heide’s Tower of Flame) head past the first groups on enemies until you are at the bottom of a long flight of stone steps, go up to the first level and into the building on your left. Up the stairs in here to find Lucatiel leaning against a wall. (NOTE: Originally she was by the bonfire just before this area so don't miss her).

Now you can work through No Man’s Wharf but don’t enter the fog gate on the ship at the end. Instead examine the wooden docks just before the ship and (if you are human NOT hollow) you should find a summon sign for Lucatiel on the floor. It’s better to kill all other enemies before the boss to make your life easier. You can also lower a shortcut just behind where Carhillion of the Fold sits back to the start and find a second summon sign which will make the fight a doddle. Now take her to the Flexile Sentry boss fight and kill him – MAKE SURE SHE SURVIVES THE FIGHT OTHERWISE IT DOESN’T COUNT, this holds true for every time you summon her. She needs to survive 3 fights out of the four available. To make things easier you can buy the Redeye Ring from the cat in Majula for 6,700 souls as this will force the enemies to focus on you rather than the NPC.

After you kill the Last Giant you will get the Soldiers Key, you can use this to access a door behind the Ironclad Solder opposite the ballista room. Up here you will find the fog gate leading to The Pursuer. You need to defeat him and then go into the room behind him to find a birds nest, examine it to be taken to the Lost Bastille. From where you land you need to head to the tower opposite to find Lucatiel. Talk to her until she repeats herself. You can also reach her if coming via No Mans Wharf. From the Exile Cells come onto the battlements, down the ladder and then left to a pull switch that opens a gate. Through the gate and up the stairs will get you to her tower.

Be sure to summon her for the Lost Sinner fight in Sinners Rise (her summon sign is in the same room that the elevator stops in on your way down, again kill the enemies leading to the fog gate before activating her).

You will find her again in Earthen Peak (she has once again moved from where she was in the original Dark Souls 2). As soon as you enter Earthen Peak proper (and the words actually come up on screen) look to your right, after killing an enemies of course). You will see a little passage in the bottom right corner that you cannot use until you stop the poison by burning the windmill further up the tower. Do this and REMEMBER TO COME BACK HERE AND SPEAK TO HER. Then talk to her until she repeats herself again.

You can summon her for the Smelter Demon fight in Iron Keep (it’s an optional boss just after the bridge you can lower) and her sign is right outside. This is a tough one to get her to survive but it can be done, I especially recommend summoning another player to help out to make things easier.

Next you will find her in the Black Gulch. When you enter, go left into the bonfire room and light a torch. Now go back out and edge along the cliff right outside until you see a platform you can drop down to (near a cluster of three poison spitting statues). Do so and you will find Lucatiel leaning against the wall in this passage, talk to her repeatedly. Then continue onwards but DO NOT go in the fog gate, you need to find the hidden bonfire. If you hug the wall to the left (if your back is to the fog gate) and follow it until you reach the cliff edge, you can then go left on a narrow path to the bonfire room (IMPORTANT NOTE: There is a statue blocking this entrance so be sure to have a Fragrant Branch of Yore on you - you get one on a corpse just before entering Black Gulch, so don't use it anywhere else) and Lucatiel’s summon sign. Now you can fight The Rotten and make sure she survives. 

You can also find her in the Crown of the Ivory King DLC, in the Grand Cathedral, to be summoned for the boss fight there which would count towards the requirements if she survives.

Continue with the game until you reach Aldia’s Keep and she will be in the first little hut by the bonfire. DO NOT SPEAK TO HER UNLESS SHE HAS SURVIVED THREE BOSS FIGHTS as she will vanish after your conversation is done. If you found her and she survived the required number of fights you will get her equipment and the achievements.

If you missed a fight or she died (if she dies before the boss is dead then you can let it kill you and summon her again, though obviously you need to use an Effigy or Life Protection Ring to be human again) you have two options. Either try again on NG+ OR use a Bonfire Ascetic at the relevant bonfire (No Mans Wharf, Threshold Bridge, Sinners Rise, Hidden Bonfire – Black Gulch) to make the boss respawn and attempt the fight again. If you do the latter the boss will be tougher and (in the case of the Sinner and Flexile Sentry) have allies. You can theoretically use this method to get all three boss wins on ONE fight if you feel like it (so defeating the Flexile Sentry three times on one playthrough via Ascetic use would be fine). But you still HAVE TO speak to her in the required places and have the wins before talking to her in Aldia’s Keep.