We are just a few days before the end of the halloween event and a lot of people still unable to finish the Inner Chamber - Nightmare.

We all know how much much bullshit this event was at its first days, but after the nerf it becomes pretty manageable. This still don't mean you can easy trample over it with 5★ bases and TMs. Instead it requires a good deal of preparation and a tight strategy. Every attack from the boss can be neutralized to an extent, and if you do things properly, you go through it with little dificulty.

Requirements

First I will list the requirements, while all those requiriments are attainable by the average player (no 5★ base or specific unit required) there is a good deal of things you need to acheive this goals.

AoE Cover Tanker

You need an AoE cover tank that can take all the physical damage:
hnichol
Be it through evasion or sheer endurance the unit should be able to take all the AoE damage all normal attacks (beware for criticals). 100% evasion is the safer route, but since gearing for that might be hard you can go with a normal tanker. You will need a lot of HP and DEF (~9000 HP and ~400DEF before buffs), if you have units that can help with damage mitigation (Veritas of the Dark, 9S or Pod, Zargablaght...) it will be another help on keeping the tanker alive.
This is probably the most strict requirement, as you need an AoE cover to not risk the other party members to die for the pjysical AoE.

ATK/MAG Break

A 40% or more MAG break is needed to reduce the MP drain from Soul Syphoon. A 40% ATK break or more is needed if you're not using a evasion tank. Notice that this should be on units that can peform other roles beside just breaking to not overload your other units. Good options for this:
hnichol
You don't need the MAG break for the 100%~60% phase of the fight and you don't need ATK break if you're using an Evade AoE tank, but whenever you need it, they need to be up all time.

Instant MP Recovery

Refresh type effects aren't good for this, you need effects that cure MP immediatedely on demmand. Items come pretty handy, specially the AoE MP recovery scrolls from expeditions. The following units can provide enough AoE MP recovery (higher than 40MP, lower value might not be enough for many damage dealers to keep using their attacks).
(WWFina, why no WWFina on reddit?!?)
Even if you have a unit capable of recovering MP, bring all MP recovery items you can (Elixir, Turbo Ether, Mega Ether, Chocolate, MP Scrolls...)

MP Drain

You need to be able to Drain all the boss MP on the first turn, this is possible with high damage dealers using MP absorbing skills (Lance, Osmose Blade, Osmose, Duskblade...). You also want one of the two damage dealers to be dedicated on Draining MP once the boss start recovering back. Having the boss at 0 MP on the start of every of it's turns is of utmost importance if you want to be safe, as it will nullify all of the boss Dark Attacks. Any high ATK unit Lance (Odin), or high MAG DC mage with Osmose (Ramuh, Diablos) can fill this role.

Fire Resistance

Fire damage is the main threat you fight once get into the 40%~0% phase. Once the boss uses Eternal Torment or Soul Syphoon, he won't use Dream Stealer, Shadow Storm, Accursed Flames for the rest of the turn. But he can still use Hailfire. And worse than that, he can spam Hailfires, so a Fire Resist of ~80% from equips is needed to be safe unless the unit has a very high SPR. You also need to keep fire resistance buff up all time, since the boss will reduce fire resistance by 50%.
For the non TM gears with fire resist you have Ifrit (50%, Esper) Flame Shield (50%, Craftable), Flame Mail (50%, Craftable), Cat-Ear Hood (20%, Star Quartz), Dragon Helm (20%, Quest), Dragon Mail (30%, Black Key), Dragon Shield (30%, Quest), Hero Sheield (20%, Trial), Ifrit's Claw ( 20%, Trial), Red Ring ( 15%, Flamering Key), Red Jacket (30%, Aurora Key), Adaman Vest (50%, Event), Imperial Helm (15%, Event)
Units that can provide high fire resist are:
hsakura
You can also Equip Carbunle on a unit for barfira if you have enough gear with fire resist.
As long you have a total of at last 130% fire resist while counting the buffs, the fire damage will pose little threat.

Confuse Immunity

Confuse immunity is also very important, as the boss has high chance of causig that to the whole party. The simplest route is using Aurring Protection from Lakshimi, specially considering that you would be using her if you didn't do the esper mission. Alternatively you can gear for confuse immunity, although there are little options for that (for the non-TM items, only Black Choker, Grand Helm and Confuse Resist materia) and you also need to find a friend unit (if you're using one) that also has confuse immunity. I recommend sticking with the esper unless your units actions is already too cluttered.

SPR Gear/Buffs

You can't fully negate the damage and drain from Soul Syphoon, but you can mitigate it by having a high SPR. SPR buffs are highly recomended if you can fit a unit that has high buffs. If the boss has 40% MAG break and the target has ~500 SPR, the boss will take ~90 MP from the target. Not a vital component of the team, but if you can fit, go for it to make things easier.

Damage

Just mention it because the boss can heal ~1.500.000 HP very frequently after the 40% treshold, so you need to keep a high damage while also being able to drain his MP.

Things that you shouldn't worry much

There are two things that people are used to worry, but that don't matter much: the DEF/SPR break and Dark Resist The break was reduced to 20% break after the nerf and is nothing you should worry too much. The Dark damage is also a non-factor as long you don't let the boss start its turn with MP.

The Fight

The fight is actually pretty straight forward. The boss has 3 phases: 100%~60%, 60%~40% and 40%~0%. There is also a RNG factor since none of the special attacks

Turn 1

Drain all the boss MP with your damage dealers and turn ON the attack break (if using non-evasion tank)

100%~60% Phase

This phase is very easy, just be sure to not forget about your tanker and things will go very smooth. There is actually another treshold at 80%, but this one makes little difference if the boss MP is drained.

60%~40% Phase

During this phase he will start using Soul Syphoon and Horror. Before going bellow the 60% HP there are some things you shouldn't forget:
- If you're not using Confusion resist gear, remember to Activate the resist buff.
- Break the boss MAG if you didn't do it already.
- Buff the party SPR if you didn't do it already.
- Buff the party fire resist, from now on, Hailfire can deal damage if used after Soul Syphoon.

Try to not let it build it's MP back.

40%~0% Phase

During this phase the boss start using Eternal Torment and Shadow Storm. You shouldn't worry about Shadow Storm, it will not deal any damage to you as long you keep the boss MP at 0.
Always drain the boss MP when he uses Eternal Torment. DC mages have some advantages on this phase, since they can Osmose and deal high damage on the same turn. If you can fit an Osmose on a 3^rd unit, you can make this phase faster by draining the boss MP on the turns he doesn't use Eternal Toment. Even a 200 MAG unit can drain all the boss MP when he uses only Soul Syphoon.

From now on, the only issue you might face on this phase is if your Damage is too low and you can't race his recovery or if you mess your buffs rotation and forget to keep up something important.

EDIT: The Fire Resist issue

I've seen some people complaing about still having issues on the 40%~0% phase. If you're having a hard time on that part, the issue is most likely the low Fire resist. Here some things to consider:

• Flame Armor and Flame Shield: Both gives 50% fire resist, are craftable, they can be equiped by anyone using Equip H Shield (Charlotte TM) and Equip H Armor (Firion TM). Those who participated Bahamut Raid should have some copies of Equip H Armor.
• Equip L Shield (Aurora Key) allows you to equip Dragon Shield (30% Fire resist, Aurora Key)
• Red Jacket (Aurora Key) is a nice and often overlooked gear that can be used.
• Adaman Vest (FFXII Event - Henne Mines) is an excellent armor for your tanker, with 50% Fire resist and 10% HP.

Example setup:
- Damage Dealer 1: Flame Mail Imperial Helm Ifrit's Claw = 95%
- Damage Dealer 2: Flame Mail Dragon Helm = 70%
- Tanker: Adaman Vest Flame Shield = 100%
- Battery: Cat Ear Hood Ifrit = 70%
- Support: Red Jacket Dragon Sheild (Equip L Shield) Red Ring = 75%

This is a non-TM setup, but if you have spare Equip H Armor or Equip H Shield its easier. The units that can't have Fire resist too high (50%~75%) should be geared for SPR. Simply don't go with anyone bellow 50% unless they have very high SPR.

Also, always keep fire resist up to negate the fire imperil.