In order to get to the Living Failures, the Hunter must go through the Research Hall and rotate the stairs to gain access to the door that leads to Lumenwood Gardens.

When first entering the arena, only one Living Failure will be present. After a few seconds, another one will appear. This pattern will continue until a maximum of 4 are in the arena. When one dies, another will spawn to take its place.
Although there are four Living Failure at any given time, they all draw from the same health pool. However, if an individual is damaged enough it will die.

Living Failures self-organize, always some Failures will employ melee attacks, while others summon Arcane volleys.
The melee ones will usually follow you around, though allowing them to attack and dodging out of the way will slow them down greatly.
The casters can perform 2 different arcane attacks: Magic Orb and Homing Bolts.
The casters only perform melee if the player is in close proximity. They can either use Headbutts, flailing their arms (twice) or slams.
During phase 2 slams can be accompanied by an arcane explosion.
This boss has two phases:

Phase 1:
The Living Failures will start spawning one by one as the fight progresses, until all four are present.
They will use abilities depending on their type (melee or caster), targeting the player.
Phase 2:
When the boss is at about 60% health, phase two will start.
All of the Living Failures will stop and raise their hands toward the sky, using the Meteor Storm ability.
The room will get dark and the sky will turn into the cosmos, after which blue meteors will rain down on the field.
The Failures will not retaliate when attacked during this. (Stunning one without killing it may take it out of the "cast")
Killing a Living Failure during this attack will still make a new one spawn, which will attack you normally.
If obtained earlier, the Loch Shield is especially helpful during this phase, as it boasts a high arcane resistance. When equipped and blocking, the player can survive multiple meteor strikes with little damage taken, as long as the player has enough stamina.
The boss will now have access to this attack and can perform it any time during the remainder of the fight.
The attack does not require any specific amount of Living Failures, as long as one is alive the boss can perform this attack.

You can summon Yamamura for this fight. (Must be killed in his cell to do this)


Strategy
To keep the battle simple, focus on one individual at a time. Killing it will cause it respawn elsewhere, giving you a few precious moments having to deal with only 3 of them.
Meteors can be easily avoided. If the you run counter-clockwise, they will be safe. Likewise, using the Lumenwood tree as cover will protect you from any damage. If both of these cannot be done, the Loch Shield can be used to absorb a large amount of the damage, giving you the opportunity to relocate to a safer position. Another easy way to avoid the meteors is to run into the doorway of the opposite side of where you enter the boss room. It's very difficult for them to hit you there.
Always try to attack whichever Failure is the most isolated. Usually, the 2 melee Failures will follow you while the casters keep their distance. This makes the casters easier targets, especially since the time it takes for them to channel their spells leave them defenseless.
Summoning Yamamura can make the fight a whole lot more manageable, since he can distract, and counter the failures. You can both use him to distract two or more of the failures, so you only have to take care of two of them, or you can have him distract them, while you sneak up behind them, and backstab+visceral them (Keep in mind, that while Yamamura can heal, and take a bunch of damage, leaving him to three or four of the failures alone for too long, will result in him dying). If you treat Yamamura with somewhat care, he has enough health and healing, to last the whole battle. So make sure to check out how to unlock him, it's quite easy!
Avoid being surrounded at all costs. Try to keep in mind the relative position of each Failure, as it is easy to lose track. This leads to occasions where you would dodge away from one's attack, directly into another that was out of sight.
The meteor spell that they cast always comes from the same location. This location is from the sky, past the balcony at the far end of the arena when looking from the entrance and to the right.
The meteor attack presents a very large opening for backstabs, but be cautious whether or not you'll be safe from the meteors. However, the visceral animation grants a brief window of invincibility, so use that to your advantage.
Other opportunities for backstabs are after they perform a Headbutt, during their spawn animation, and when the casters channel their arcane attacks.
The sides of a Failure are a good place to attack, as you will be safe from their Headbutt and Slams. The back would be optimal, but it is extremely difficult to stick there, especially while playing solo. Keep in mind that you can still be hit by Flailing and Swipes.
Using a heavy, hard hitting weapon like Kirkhammer or Whirligig Saw and using a hit 'n' run tactic by using the running R2 attack is very effective. It staggers the Failures and interrupts their attacks. This can also be accomplished by performing a jumping attack with a large weapon. If you see 3-4 Failures close to each other, you can fairly safely hit them all and do massive damage to the shared health bar.
They are weak to Bolt, so equip an appropriate Gem Stone or have Bolt Paper ready.
Because of this, the Tonitrus is very effective in this fight.
Rule of thumb for counter shotting is that when they swing their arms left (your right) they always swing them back right (your left). That second swing is very easy to parry.
After killing a Failure, while the death animation is still playing you can usually get a single attack in, which is helpful if you've been attacked but can still recover health.
The Shaman Bone causes the Living Failure to turn against one of the others for a duration of time. This allows you to reposition or heal as 2 out of 4 of them will be occupied.
Triggering a death animation on one of the Living Failures immediately triggers the spawn animation for another somewhere in the arena. The animation has the Living Failure begin in a fetal position and slowly raises to a stand. During this animation, it has no means to defend itself allowing for easy damage or backstabbing.
Using the upgraded Whirligig Saw's L2 during this animation will almost always guarantee a kill before the living failure can attack. If done early enough, this will prevent more than 2 Living Failures from spawning, and you'll only have to worry if the second one tries to engage in physical attacks.
The Living Failures have a specific spawning rotation, at four different corners of the arena, in a counter-clockwise order, starting with the left corner near the door. (from where you enter)
If you are quick enough and have enough damage it's possible to simply camp a single spawn point without allowing the boss to do anything at all, killing the new spawn repeatedly.
Rapid poison is a major weakness of this boss but it works weirdly. It seems that when Rapid Poison triggers it drains HP from the common health pool and not from any individual Failure. So basically Rapid Poison can't kill any single Failure but can deal the final blow to the whole boss fight, killing all Failures.
If a player doesn't kill any individual Failure, they can't enter the second phase and summon meteors. So with Rapid Poison via top tier Dirty Gems players can kill the boss without ever seeing the phase two. Best dirty gems can be farmed from Wrathful Stingers in the chalice dungeons.It takes around 7-8 hits with a weapon that has Dirty Bloodgems on it to cause Rapid Poison but these hits can be done to any Failure, not just one.