While stage 4 looked threatening but wasn't too bad in execution, stage 5 might look safe in its initial resemblance to stage 2, but is far more dangerous than any area you've been through to date.
To progress through the dungeon, you'll need to do several things. First, you'll begin with the passages on the left side (The top leads to dead-ends), and eventually emerge on the right behind a switch. Pull it to create a shortcut back to this point, and then resume travelling to emerge on the left corridor again behind the last switch for another shortcut. Your objective finally is to traverse that last area, head down into the basement, and find the boss.
It's not that easy though. First off, you'll need to find two golden keys as you travel - one is located between the first and second shortcuts, and the second is in the last segment. You'll need the first key to open up the basement, and the second one to reach the boss, so be sure to keep an eye out for them. There's also two silver keys fairly well hidden down here - if you can find them both, you'll be able to open a couple silver chests in a room right next to the boss area. Just getting that far though, you'll also have to navigate through hidden passages. Keep an eye on the walls, and you should begin to notice the signs of a hidden route. Finally, you'll also have to look for unlit candles - check them to light them, and they'll light up the dark panels on the floor, unlocking the occasional sealed door in the area.
If navigating this place isn't enough of a hassle, many of the enemies you came to hate in the previous stage are back (Red gators, hellhounds, even the instant-death packing stones), as well as new things that will make your life hell. Two-tailed cats can charm party members when attacking - or charm the whole party at once. Even more fearsome, the mind flayers boast high resistance to all magic, are capable of paralyzing your whole party, and their regular attack can cause instant death. Even a single one of these can destroy your party with a little bit of luck on their side.
It doesn't get easier as you go, either. Near the end, spiders will begin to show up - they have a column attack that can reduce all of your stats at once, and resist all forms of physical assault. More dangerous than them are the large bird enemies, who can use the same fire breath as the hellhound; more importantly though, they get two actions, and at the start of a round (barring any very fast actions, such as Alice's defensive spells or a Last Word), will cast a spell that does 0 damage, but has a high chance of cancelling out the casting of any character, negating their action that turn.
Between the length and design of this dungeon, and the difficulty of its enemies, be ready to rage a little before you finally succeed.
Boss: Satori Komeiji
After the long challenge that was the Palace, Satori really isn't so bad. Her HP count is pretty large, and she has no weaknesses (nor resistances, actually), but she's also not too dangerous. Satori's main form of offense is to use status-inflicting spells. She likes to debuff the party's stats, and cause other such ailments, so Sanae and Byakuren's RES buffs could prove useful to keep them from triggering.
Killer World: Once again Sakuya proves herself to be exceedingly useful with this ability. When she begins to regen 100hp, use this bad boy to ensure Satori has one bad day. As per usual, make sure to be fully healed/buffed before unleashing it. You will also be pleased to know that Satori will STILL use healing spells on herself, without realising Killer World is in effect!
Once a couple turns pass, however, Satori changes up her strategy. She will announce an elemental shift at the end of the turn, and roughly every third turn thereafter, and align herself with the indicated element. At this point, she gains 100 HP regen, and begins casting elemental spells. Once she's in an elemental phase, she is weak to the opposing element, and highly resistant to the element she claimed - but is still evenly vulneurable to all other forms of damage, so don't worry that you'll have to exploit her weaknesses. Still, Marisa, Nitori, and Patchouli make good choices for this battle to take advantage of her elemental vulneurabilities.
It's worth noting that on the last turn of her Earth-elemental phase, she will attempt to petrify the whole party. This could potentially instant-kill your party, so try to resist it with the proper accessories or by having RES buffs active when it occurs.
If you can't read Japanese, try to memorize the elemental symbols (or screenshot a status screen showing them). The first symbol in both the effect announcement, and in the dialogue right afterwards, is the element she has chosen.
Ultimately, any decently levelled party capable of taking care of itself and dealing damage should be completely capable of taking down Satori, since her elemental changing really isn't too restrictive to you, and if you're using Alice's anti-elemental shields, it will make her even easier. Just be ready to deal with some irritating status effects (Reimu's handy here), and Satori might just be the easiest thing you've faced this whole stage.
After this battle, Satori will join the party.