Operation Babel: New Tokyo Legacy

After completing all post-game missions and assignments, you'll get the assignment "Embryo Core Accident". The 7H will once again reappear in the Embryo. The 7H are even stronger than before. Once they are defeated, you will fight Soul Eater and Soul Rebirth at the portal. Once they are defeated, the portal will take you to Lethal Hall.

Lethal Hall is a dungeon with 10 battles. There are no random battles here. If you leave this dungeon, you have to start from the first battle again. If you plan on conquering this dungeon, I recommend stocking on Power Recorders, any Decoy items, Remember Guts items, Pandora Boxes, etc. You can also equip items that lower Variant's levels to make things easier. If you have a Conjurer, you can contract the Clone Yuyu and Clone Nono from the first Lethal Hall fight to makes things much easier. After every battle, you lose some Unity Rating, so it's best to try to finish each fight with as much Unity Rating as possible. Note that the first time you finish the first wave of the fifth fight, you lose your contracts and buffs, but it won't happen in future fights, so if you really want to have Clone Yuyu and Clone Nono for later fights, you should leave after finishing the fifth fight for the first time, and start Lethal Hall again.

First fight
Clone Yuyu, Clone Nono, Nipple Bits (4), Capture Fly (4).

The Nipple Bits and Capture Flies are vulnerable to Crushwork/Multi-Crush. The Clone Yuyu and Clone Nono can be contracted, and I highly recommend contracting them if you have a Conjurer to make things easier. You can equip items that increase Variants' levels so you can contract even higher leveled enemies. If you have already used your Embryo Key, this fight will award you one, so you can always travel quickly back to the Lethal Hall.

Second fight
Golden Rock, Wyvern Beta, Wyvern Alpha, Rock Bit (4)

Rock Bits are vulnerable to Crushwork/Multi-Crush.

Third fight
Master Maiden, Princess Goblin (2), Zombie Murphy (2), Glamorous Maiden (3)

Glamorous Maidens and Zombie Murphies are vulnerable to Crushwork/Multi-Crush.

Fourth fight
Joker, Scryull, Algol, LXXII Pillar (3)

Fifth fight

This is a two part fight. The first time playing through this, between battles, there will be a cutscene, and you lose all of your buffs and contracts. If you revisit the dungeon later and fight this again, there won't be a cutscene and you won't lose your buffs/contracts in between battles.

[First wave]

Reuse Y Wizar, General Borg (2), Master Borg (3), Reuse Pulsar (4)

Master Borgs and General Borgs are vulnerable to Crushwork/Multi-Crush.

[Second wave]

Walpurgis, Thanatos, Embryo Wizar (2), Super Goblin (2)

Super Goblins are vulnerable to Crushwork/Multi-Crush.

Sixth fight
Dreadnaught, Buster 2007 (3), Buster 2003 (3), EQI Monohorn

The Buster 2007s and Buster 2003s are vulnerable to Crushwork/Multi-Crush.

Seventh fight
Buccaneer, Fuhrer, Voltrus

Eight fight
Final Daughter, Shinma, Perfect Daughter (2), Daughter Guard Amatsu (2)

This fight can be painful, because they can remove your buffs easily. Even if you use Anti-Default, they might cast buff removal moves more than once in a turn.

Ninth fight
Clone Akashic, Replica Z Wizar (2), Replica Feather (4)

Tenth fight

This is a three part fight.

[First Wave]

Hikaru, Kieru, Titania (2), Hyper Jericho (4)

The Hyper Jerichoes are vulnerable to Crushwork/Multi-Crush.

[Second Wave]

Vanishment, Kieru, Eraser Grid (3)

[Third Wave]

D.E.M, Deus Gear (3), Deus Gear (4)

There are three rows of enemies in this fight. Despite that, this is not much that much different from the other D.E.M fights during the story, though it's at a higher level.