Lost Sinner
The Lost Sinner is one of four mandatory bosses you must defeat to unlock the gate of the Shrine of Winter. You are required to pass through the Shrine of Winter to reach Drangleic Castle, where the final boss can be found.
Speedrun through Sinner's Rise NG+7, Dark Souls 2 SotFS
The Lost Bastille
Start from the Servants' Quarters bonfire. Ahead should be a wooden door that leads to a T-junction. Open the door and take the left path to find the Bastille Key on a corpse hanging from an edge. Watch your back because as you loot the key a Royal Swordsman would emerge from a tower and run up to you. Defeat the Royal Swordsman, head towards the tower, and ascend a flight of spiral staircase to find a room filled with columns of wooden barrels. An Undead Citizen is sitting behind a row of barrels, while another Undead Citizen is sitting near a lever. Head for the enemy near the lever and defeat it as quickly as possible. The other Undead Citizen would self-explode. Pull the lever, wait for the grate to slowly reveal the path to another grate, behind which sits an Undead Citizen. Approach the Undead Citizen and then run away, waiting for the enemy to self-explode and for the second grate to slowly reveal a passage. You will find another Undead Citizen x 2. Throw an Alluring Skull to distract these enemies, then dispatch of them as quickly as possible. A lone Undead Citizen is easily manageable, but a gang of them can easily kill your character because their self-explosion has a rather large area-of-effect.
Sinners' Rise
Head up a flight of stairs to find an Undead Citizen sitting between a gun powder barrel and a lever. Throw an Alluring Skull to distract any Undead Citizens in the area, kill the sitting Undead Citizen, throw another Alluring Skull, and pull the lever to reveal the path to Sinners' Rise. Defeat any remaining Undead Citizens you find, otherwise they would follow you to gang up on you. Around a corner you should find a Royal Swordsman guarding a corpse that holds Radiant Lifegem x 8. Defeat the Royal Swordsman and make your way towards the Undead Jailer ahead to fight him. Enter the mist and climb a ladder to light The Saltfort bonfire.
Descend the ladder, or equip the Silvercat Ring and drop down onto a platform where you should land behind the Undead Jailer's back. Equip Havel's Greatshield+5 and drop down again onto an elevator that should take you all the way down. Use the shield to block enemy bolts and attacks while you stand on the elevator. This is a long elevator ride. Use the time to organize and unclutter your inventory such as discarding any items you collected along the way and do not need.
At the end of the elevator ride, send the elevator back up in case your character dies and you must descend again. If you exhausted Lucatiel's dialogue at The Lost Bastille, you should find her summon sign near a lever. Do not summon her yet; wait until you are ready to fight the area boss.
Ahead is a narrow watery passage that leads to a grate. Do not head for the grate. Various strong enemies are ambushing you on the left side of the passage. You want to defeat these strong enemies to clear a safe path for yourself and your summoned phantoms. The first strong enemy is an Enhanced Undead. Lure the enemy into the room with the elevator, which you should have sent back up already. Use the large square hole in the room to create some distance between your character and the enemy. Spam Crystal Soul Spear at the Enhanced Undead. You might also want to poison it with Poison Arrows. The second strong enemy is a Flexile Sentry. Lure it out, then head back into the room with the elevator. The Flexile Sentry cannot enter the room. Take the opportunity to spam Crystal Soul Spear and/or poison it with Poison Arrows. The third strong enemy is another Enhanced Undead. Apply the strategy you used against the first Enhanced Undead.
Further ahead to your left should be a red phantom Undead Citizen and to your right is a regular Undead Citizen. Defeat these enemies, being careful to avoid them self-exploding near your character. Take the first right staircase to find another Undead Citizen sitting next to an iron door and outside the room where the summon sign of Sellsword Luet is found. Defeat the enemy and use the Bastille Key to open the door. You should have collected the Bastille Key near the Servants' Quarters bonfire. Drop down or climb the ladder down. You should find red phantom Undead Jailer x 2, each guarding an iron door that leads to a giant oil lamp. Defeat the red phantoms, open the iron doors, and light the giant oil lamps to increase the visibility within the boss arena. Use a Flame Butterfly to light your torch because the nearest bonfire is a long journey away. A corpse at the bottom of the left iron door holds a Radiant Lifegem. The passage to the boss arena should be safe now. Go ahead and summon Lucatiel and Luet, and enter the mist with your gang. You would not be able to use a Brightbug while a summoned (or red invading) phantom is in the area. Consume a Brightbug before summoning a phantom.
Lost Sinner
Gang warfare 3-v-3 The Lost Sinner NG+7, Dark Souls 2 SotFS
The Lost Sinner by herself is not difficult. Roll through her attacks or use Havel's Greatshield+5 to block. Position your character behind the boss and get in one hit.
The true difficulty of the fight is revealed in phase two, triggered when the boss' health bar has decreased by approximately one-third. At the beginning of phase two, red phantom pyromancer x 2 would burst through two iron grates and enter the fray. If you fight the boss by yourself, the fight would now be a 3-versus-1 gank. You can summon Lucatiel and Luet to even the numbers, making the fight a 3-versus-3. Concentrate on defeating the pyromancers as quickly as possible, then focus on the Lost Sinner herself.
Summon Lucatiel and Luet to make the fight significantly easier than entering the boss arena solo. Let the boss be distracted by your summoned phantoms, while you spam Crystal Soul Spear at her. Wield the Blue Dagger in the left hand to boost your magic damage output as you cast sorcery. Maintain some distance away from the boss so as to keep her attention on your summoned phantoms. Luet alone is sufficient to distract the boss while you spam Crystal Soul Spear. Beware that it is possible for Luet to die. Get rid of the red phantom x 2 as quickly as possible and prepare to possibly fight the boss one-on-one.
Defeat the Lost Sinner and open an iron gate to find an iron chest to your left. The treasure chest contains a Fragrant Branch of Yore. This Fragrant Branch will be used to Unpetrify Rosabeth who blocks the path to Shaded Woods. This is a mandatory use of a Fragrant Branch. There is no other legitimate way to access Shaded Woods. Alternatively, ignore this iron chest. You will collect a Fragrant Branch near the entrance to the Black Gulch. Return to Majula, level up with the Emerald Herald, and purchase all Brightbug x 3 from Melentia.