Ruin Sentinels
The Ruin Sentinels are an optional boss. The only reason to defeat them is that they block the passage to the Bastille Key, the item required to increase your visibility within the arena of the Lost Sinner. The boss arena of the Lost Sinner is usually rather dark. The Bastille Key opens two iron gates, each blocking a giant oil lamp which you can light to significantly increase your visibility while fighting the Lost Sinner.
The Lost Bastille
Speedrun through The Lost Bastille NG+7, Dark Souls 2 SotFS
Start from The Tower Apart bonfire. One of the two iron chests contains the Dull Ember, which can be given to Steady Hand McDuff to use in the infusion of weapons. Use the services of McDuff to infuse any weapons you want, but you should have already done this prior to starting the challenge run.
The most dangerous enemies in the area are the Undead Jailers. They are weak against lightning. Buff your weapon with Sunlight Blade to fight any Undead Jailer. Crystal Soul Spear is also effective. Defeat the very first Undead Jailer you find ahead of you, otherwise the enemy would harass you while you attempt to open the iron door and while you exhaust Lucatiel's dialogue. Lucatiel is leaning her back against a stone wall. Exhaust her dialogue to unlock her summon sign outside the arena of the Lost Sinner. From NG+1 onward, the fight against the Lost Sinner has a second phase where red phantom pyromancer x 2 would enter the arena after you have drained approximately one third of the Lost Sinner's HP bar, effectively making the fight a 3-versus-1 gank. Having at least one summoned phantom would take some pressure off you. Exhaust Lucatiel's dialogue and search for another Undead Jailer roaming far ahead near Royal Swordsman x 2. Defeat the Royal Swordsman x 2, but ignore the Undead Jailer. Note that Sellsword Luet is also available to be summoned for the Lost Sinner fight. Luet is more than capable of distracting the Lost Sinner and the red phantom pyromancer x 2. It is safe to ignore Lucatiel.
Use a Fragrant Branch of Yore to unpetrify an enemy blocking an iron door that leads to the arena of the Ruin Sentinels. You should have purchased a Fragrant Branch of Yore from Melentia. Defeat the enemy, open the door, and descend to the building's lowest floor.
Take a detour to the boss arena because the quickest path is littered with many Royal Swordsmen who can easily gank you to death. Use the stairs to go to the building's lowest floor, where you would find a lone Royal Swordsman guarding a ladder. Defeat the Royal Swordsman if you want, but it might be easier to lure him away from the ladder, then dash to the ladder and ascend to the floor where the boss arena is located. Do not worry about any last minute preparation. Climb the ladder and run to the fog wall, but do not enter yet. As long as you stay up close to the mist, the Royal Swordsmen would not approach you. Take your time to do any last minute check of your armament.
Equip the Silvercat Ring to reduce the fall damage up entering the mist. You would be dropped onto a platform where a Ruin Sentinel stands guard. Dropping off this platform would also incur fall damage. The Silvercat Ring can reduce any fall damage. On the other hand, you might want to reserve the ring slot for a ring that boosts your damage output and instead equip the Flying Feline Boots.
Ruin Sentinels
Sorcerer versus Ruin Sentinels NG+7, Dark Souls 2 SotFS
Enter the mist and be dropped onto a platform guarded by a Ruin Sentinel. The other Ruin Sentinel x 2 are waiting and watching from another platform at the far end of the arena. Defeat the lone Ruin Sentinel as quickly as possible while remaining on the platform. This is a fight against time. The longer you drag out the fight, the higher the chance of another Ruin Sentinel entering the arena and leaping onto the platform where you are fighting the first Ruin Sentinel.
An easy strategy is to block with Havel's Greatshield+5 and cast Crystal Soul Spear. Soul Spear is an effective alternative. As you are blocking an attack, strafe behind the Ruin Sentinel and cast Crystal Soul Spear. Equip the Clear Bluestone Ring+2 to shorten your casting time, the Sorcery Clutch Ring to boost your magic attacks, and the Southern Ritual Band+2 to grant you an additional three Attunement slots. Consume a Brightbug before entering the arena to boost your attack and defense. The best time to spam Crystal Soul Spear is when the Ruin Sentinel you are fighting drops off the platform onto the floor below. While locked onto the Ruin Sentinel, spam Crystal Soul Spear. You should be able to perform at least three casts before the Ruin Sentinel decides to jump onto the platform again.
Having defeated the first Ruin Sentinel, the second Ruin Sentinel would drop onto the floor of the arena and make its way to the platform on which you stand. Lock onto the second Ruin Sentinel and spam Crystal Soul Spear. You should be able to perform a minimum of three casts before the Ruin Sentinel leaps onto the platform. Back away to regenerate your Stamina and prepare to block any incoming attacks. With the second Ruin Sentinel on the platform, fight it as per the strategy you used against the first Ruin Sentinel. Use the same strategy against the third Ruin Sentinel.
The success of the above strategy hinges on two factors:
- Stay put on the platform onto which you originally landed. Dropping onto the floor of the arena would trigger all Ruin Sentinel x 3 to enter the arena floor, turning the fight into a 3-versus-1 gank. If you get knocked off the platform, or you accidentally drop off the edge, reset the fight by warping back to the last bonfire at which you rested (or let the Ruin Sentinel x 3 kill your character). It is OK to fight the last remaining Ruin Sentinel on the arena floor. However, fighting on the platform should be your first choice as it allows you to cheese the enemy.
- High damage per second (DPS). Crystal Soul Spear is the best spell for dealing with each Ruin Sentinel individually. You want to manipulate the Ruin Sentinel you are fighting to drop off the edge of the platform, giving you an opportunity to spam Crystal Soul Spear. For high melee DPS, power stance with two great hammers such as Old Knight Hammer x 2, Great Club x 2, or Giant Warrior Club x 2. Buff your weapon with Sunlight Blade.
Defeat the Ruin Sentinels and ascend two flights of stairs. At the top of the stairs, the exit is to your right. Search for an illusory wall to your left by interacting with the wall. Be ready to fight an Undead Prisoner who sits next to a wooden chest. Do not swing your weapon at the enemy in case you break the chest. Instead lock onto the enemy and cast Crystal Soul Spear. An alternative strategy would be to throw an Alluring Skull to lure the Undead Prisoner way from the treasure chest. The chest contains Crimson Water x 3. Each item heals you for 1,500 HP and restores many spell uses, a viable alternative to the various herbs. Find and light the Servants' Quarters bonfire. Rest at the bonfire as necessary.