Hakugyokurou is split into two levels - the heavenly upper floor, and the dead underworld beneath. You can travel between the two via the green teleporters, but you can also fall through cracked floors on the upper floor to reach the lower, which will allow you to reach some areas you occasionally couldn't otherwise.
Despite that, this dungeon is relatively straightforward. The first half consists of finding a hole to fall down that puts you beyond a barrier (There's a switch nearby to deactivate it), and getting the gold key in that area. Afterwards, the golden door nearby leads to a teleporter that lets you create a shortcut, as well as the second half of the dungeon - in which you'll do pretty much the exact same thing. There's two silver keys to find in the first half, and two silver chests in the second half, as well.
After Stage 5's enemies, the enemies here are relatively nonthreatening. The most dangerous are the dark bat and light phoenix-looking enemies on the bottom and upper floors respectively, although they're rather rare. They both get two actions per turn, and carry some dangerous moves - the bat can manipulate the field with ease and block off light-elemental attacks, as well as having a powerful dark-element breath move; the phoenix can drain HP, and has a spell that will both raise the evasion of every enemy and revive any of your dead party members under Control status, ala Orin. You can probably take these things on solo, but if there's more than one or they're in a group, it might be best to run.
This stage is the only place you can find the Peach of the Underworld item, a yellow potion-looking material used to create the second set of Unique weapons. Make sure to search it thuroughly to find a couple!
Boss: Yuyuko Saigyouji and Youmu Konpaku
Yuyuko Saigyouji's HP: 3500 (100 Regen in Dark) Youmu Konpaku's HP: 3500
A really dangerous pair of characters, this boss is well-known for causing a lot of rage. The reason is that both of this duo are extremely dangerous, and capable of killing characters seemingly on a whim. Beating them will take careful preparation and planning, as well as a healthy dose of luck.
Youmu's threat comes in the form of massive physical power - she carries several different spellcards, and some of them will nail your entire party, or make random assaults, and it's not an exaggeration to say they will tear apart anyone who doesn't have a large amount of HP and defenses. One of her spellcards, however, is limited to a small area effect, and you can divert that from your more frail characters if you use Alice in a formation that will draw the attack away. Still, you're going to need a handful of luck to survive. Taking her out before Yuyuko is advised, as whichever remains will get two attacks per turn, and you do not want to face Youmu like that.
Yuyuko can be just as dangerous, but it's for other reasons than Youmu. Her damage output is very low; however, she favours status attacks which inflict either Curse, or Death - the latter of which is obviously extremely dangerous. One solution is to use Alice to draw her fire, and equip Alice with anti-death and resistance-boosting accessories, making the chance of her being successful nearly nil; if you can boost your RES, this gets even easier. Her only actual attacks, thankfully, are dark-elemental - the danger from that is more that she will eventually begin regenerating.
Killing the two will prove a little difficult, though. Youmu packs no magical defense and a weakness to the four elements, but she's got a decent physical defense, a high evasion, and physical counterattacks, so you'll definitely want to be taking advantage of that to take her down. Yuyuko has the four elemental weaknesses as well, but she resists all other magical damage and packs an incredible magic defense stat, so not much will get through to her. Unfortunately, physical assaults will find it difficult to succeed too - she's got a whopping 75 Evasion, so you'll definitely want to raise your party's hit rate (Sanae can do this). Once you can land a hit though,s he's got no physical defense whatsoever.
The final kick in the pants with this battle is that, when reduced to 0 HP, Yuyuko will cast Saigyouji Flawless Nirvana, a spell with very high death infliction rates against the whole party! Thankfully, there are several ways to handle this - death resistance gear and resistance boosts might let you survive, but if you'd rather not rely on luck, there are other ways. Mokou's 3-star commander spell will prevent the party from dying from it, and Reimu's barrier spell can also negate death for a single turn. You can also have Alice use a spell to cover an ally - she'll be hit twice with the spell, but the covered character will survive, so if Yuyuko is the last to fall, you'll win the battle. The spell will also fail to activate if she is killed during Sakuya's Private World spellcard. It will also fail to trigger if she is killed by Poison; for that option to be reasonable, have Byakuren back up Satori by raising Satori's IND and dropping Yuyuko's RES, and then let Satori inflict poison - it'll be dealing 300 or more damage a turn, so then it's just a matter of not getting killed off by the assault while you wait.
Once you finally manage to win this battle, Youmu Konpaku will join your party. If you've recruited Byakuren and Mokou, you now have every character in the game.