This post-game dungeon's area 2 - 4 are 100x100 tile maps. It can be a chore to fully explore it. This guide will shows you the route to the Core Guardian and Nightmare. Mary Reverser and Hameln is needed for this route.
Mary Reverser and Hameln
To use this route, you need Jack's Mary Reverser and Hameln's Iron Piccolo.
Retrieve Gun Parts and give them to Haru to upgrade your Mary Gun. Mary Reverser will be unlocked after all 3 upgrades are done.
Get Prison Key 10 on Downtown Area 2 (36,22)
Unlock the door (27,28) to Area 4
Area 4 (06,38)
Move forward to Area 3 (5,45)
Warp puzzle (7,18) (Follow the warp with white stone beside it)
Get Dorm Key 1 (39,38) (hidden room, use Snow White's bomb)
Go back to Area 1 and unlock the door (40,31) to Area 2
Warp puzzle (22,34)
Area 2 (31,07)
Area 1 (14,33)
I prefer to recruit her as soon as possible (Chapter 7). Once you have Mary Reverser, go to City Streets Area 2 (28,10)
Turn back and use the Reverser to cross the chasm. Move forward to Underground Cavern Area 1 and fight her.
Facing East, use the Mary Reverser to cross the chasm.
Use the Iron Picollo.
Use the Iron Picollo.
Face South and use the Reverser.
Area 3 to 4 - Getting the Key
Grab the heart slate and use Thumbelina's Tiny Princess to shrink it.
Backtrack to Area 3.
Area 3 - Boss Fights
Rapunzel's Hair Flair.
Guardian (weak to Water).
Nightmare 1st phase (weak to Air).
Nightmare 2nd phase (weak to Lightning).
Easy Kill on Boss
(On Horror difficulty you'd need to grind more. High agility and high defense is necessary). The Core Guardian has a multi-hit skill (up to 4 times on a character) that bypass Paladin's Cover. The Nightmare will always open with Sleepy Times (AoE Sleep) & Tremor (on its 2nd fight).
For the Guardian, Physical Resist gear is useful for mitigating the damage. For the Nightmare, Andvaranaut accessory along with Physical Resist and/or Earth Resist gear. You can purchase Andvaranut from the wandering merchant in Jail Tower. Shrink it first to minimize the stat decrease.
The quick and easy way to defeat them lies in Jail Bonus Wheel buffed by Logic's passives.
Spec Snow White and/or Gretel as Logic and max out the Jail Up passives. (You can spec them as Item Meister, Mimic, and Blood chemist first if you want versatility). Make sure the Hunger and Libido bonus can be activated within one attack / lick.
With Logic's Jail bonuses, the % damage to enemy is increased and can occurs twice. On normal difficulty, the Nightmare received 22k(!) damage. Doubled, it's already 44k. It's possible to defeat it before it takes another turn.
After defeating the nightmare, you can access this area.
Cinderella's Twelve Dash.
Use the switch to open the locked gate.
Backtrack or use Reverser.
Same as before. Explore or use Reverser.
Forward to Area 4.
Hameln's Iron Piccolo then get across with Reverser or Gretel's Bread Portal.
Kaguya's Bamboo Guard.
To unlock the gate, extinguish the 3 candles with Mary Gun.