This guide is just my take on the dungeon. You don't need to do anything listed here if you don't want to, this is mainly advice for people who may be struggling with the dungeon. You may be able to beat it with 0 help from this guide, so I suggest trying it first and see where it gets you.

This guide is for the TRUE version of this dungeon, not the one you beat during the story or the one that unlocks afterward. If you want the Final Millenium Tower, I suggest checking out MaverickZealot's guide:
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2291384215
Anyway, this guide pretty much follows similar rules as theirs, it will not show item locations and it will only go over the mandatory fights in order to reach the top.

Don't like my guide? Check out another one made by DannyBoy! They actually use images in that guide so it's plenty more user-friendly than this one I think:
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2325800077
That being said, this guide will not show enemy locations on the map or their Sujidex entries, simply the names and information about the enemies that appear on each floor. If you're doing this dungeon, that means you've run through the Millenium Tower at least twice now and you have a basic idea of how it goes. Since every fight has different enemies in it, you can find your targets here by using CTRL+F.

If you've played RPGs before or have the basics of the game down to a tee, a lot of this will come off as common sense for you. Many of these apply to Legend difficulty as well, except at that point they become a necessity rather than a suggestion. You WILL need more defense on your party members and you WILL need as many health and MP items as possible.

• While the dungeon does not let you continue if you die, the boss room does. One of the main features of this dungeon is that once you die, you're sent straight to the title to start from your last save. If you die in the final boss room, however, you are given the option to continue just like anywhere else, so keep this in mind.
  
• All of the normal enemies here are lvl 99. The only exceptions are some of the minibosses. You want to be lvl 99 and have a max level job rank in order to have an easier time here. Expect to lose up to and over half a party member's health when they are attacked by a single enemy.
  [LEGEND]: They don't deal much more damage, but some enemies seem to attack more often.
  
• Bring as many health and MP restoring items as you can and make sure your party is kept up on their MP whenever possible. I didn't need any health items for my runs, but I definitely used MP items. You'll be using your abilities way more than regular attacks here. The only things you won't really need are revival items, but that's only if your team is prepared with everyone having either Soul Tether and the healer having whatever heals she has.
  [LEGEND]: Start stocking up on EVERYTHING that can possibly help you. The fights don't seem to last much longer, however some enemies do deal quite a bit more damage than normal. You'll be using your skills much more often here, and for longer.
  
• Grind the booster drinks. They help out, a LOT. On my first run through the dungeon, Zhao and Saeko had abysmal defense and it was a pain keeping them alive. After a while of grinding for the booster drinks, I got their defenses up enough that they weren't losing 80% of their health on each turn and had a much easier time (go figure).
  [LEGEND]: You will absolutely need these on Legend difficulty. They will make your life SO much easier. You WILL get one shot at the highest difficulty if you don't use any defense drinks.
  
• Try to max out your weapons. You don't need the best weapons in order to succeed, you need them so the dungeon doesn't take 5 hours to finish. Here is the list of "best weapons" I referred to.[http//%5Bhttps]
  [LEGEND]: Use fully upgraded weapons, the normal dungeon on its own takes about 45 minute-1 hour with fully upgraded weapons, and Legend mode is even longer.
  
• Make sure you have as many, if not all, summons unlocked. They aren't a necessity, save for one. Make sure you have Mr. Masochist above all else, as he will be a huge help during the final fight.
  [LEGEND]: Once you start New Game + on Legend difficulty, your Poundmates carry over, so you don't need to bother with unlocking them all again.
  
• The Management game is optional. Don't worry about it at all unless you want Eri to be present at any point for this dungeon. Essence of Orbital Strike may be strong, but it doesn't compare to how much damage you can deal with Fulminating Forecast, and at a much lower MP cost too. Every fight with the exception of the final boss battle will only have 3 enemies, so it's simply not needed.

These are the jobs and gear I had on my party when I finished the dungeon. I'm not saying you need to run this team to succeed, but just know that this team can easily run through the dungeon with few to no deaths.

[LEGEND]: With each subsequent New Game+ playthrough of the game, you get a free Quixotic Armor Set near the end of the Millenium Tower dungeon. Just add that onto whoever's next in line in your party. Personally, I always used the "use optimal gear" choice, and it's worked absolutely fine for armors.

Hero Kasuga
Legendary Hero's Bat EX EX
Quixotic Helmet
Quixotic Armor
Quixotic Greaves
Brawler God's Mouthguard
Eye of the Dragon
Stat boosted for defense and attack.

You'll notice, I'm not using any resurrection items. Feel free to use them if you need them, but the dungeon is more than doable without them.

Kasuga takes the role of damage dealer, buffer, and secondary healer, depending on the situation. If Saeko is down or unable to heal due to turn order, Kasuga's Hero's Compassion (Hero) is invaluable and able to fully heal the entire party instantly.
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Gangster Zhao
Curved Crystal Saber EX EX
Viking Helmet
Dragon Kinagashi
Dragon Boots
Diabolic Wristband
Kiwami Mouthguard
Stat boosted for defense and magic.

Main damage dealer. His attack is the highest in the game and his magic isn't something to scoff at either. Make sure he's got as many damaging moves as possible.
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Enforcer Adachi
Special Ops Riot Shield EX EX
Cyber Helmet
Dragon Kinagashi
Illegal Boots
War God's Keystone x2
Stat boosted for defense, attack, and agility.

Honestly, Adachi can be replaced with anyone, but I took Adachi as Enforcer juuuuust in case something went wrong and I needed to make sure Kasuga lives. He's the backup for the backup plan, a tank to take some damage in case I need some extra time to heal and revive people.
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Idol Saeko
Celestial Microphone EX EX
Cyber Helmet
Dragon Kinagashi
Dragon Boots
Bracelet of the Four Gods
Transcendent Insoles
Stat boosted for defense and agility.

She's the main healer and ailment remover. If anyone is hurt over a third of their health, start healing. With her base agility, she should already be going first in your party anyway. If nobody is hurt, start with Lovedrunk Typhoon (Idol). In this dungeon, don't rely on the charming, just use it for the defense reduction.

She has the only actual conscious decision made toward her survival, and that's through using the Bracelet of the Four Gods on her. For the price of 50 defense, it nullifies all status ailments, meaning that as long as Saeko is up, she can remove everyone's ailments once it is safe to do so. Believe me, this item alone is a massive help to her.

[LEGEND]: You will most likely need the Bracelet of the Four Gods on her. At the hardest difficulty, your team cannot afford to be left with an incapacitated or dead healer.
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I will reference these skills a LOT. Fulminating Forecast is an absolute MUST-HAVE in this dungeon.
"B-but Essence of Orbital Las-"
You'll run out of MP much faster while dealing less health. Get Fulminating Forecast.

• Ice Spreader (Host and Hostess). There are some enemies that are weak to water, and this is the only water skill that can be used in other jobs. It's available for both the guys and Saeko.
  
• Megaton Throw (Foreman). Some enemies are weak to fire as well, so it's good to have this on the guys just in case.
  
• Fulminating Forecast (Fortuneteller). This skill is absolutely ridiculous. It's essentially a Kiwami move, dealing extreme electric damage to three random enemies. Well in most battles, all you ever have is three enemies and many are weak to electric. All of my guys were using this ability in nearly every battle. It has a pretty high paralysis rate, too.
  
• Paralysis Prongs (Enforcer Exclusive): To make things go by faster against a single enemy weak against electricity, use this. This skill appears to be the strongest single target damaging electric move and it will deal up to 3x as much damage as just using Fulminating Forecast. Seriously, give it a try if you haven't already. This skill is exclusive to Enforcers, so you can only use this if you have one on your team.
  [NOTE]: These two skills will be mentioned a LOT in this guide, so to make things easier I will refer to the two abilities in tandem as FF/PP.
  
• Diehard Skewer (Bodyguard). Adachi and Kasuga don't mainly deal with blade attacks, so it's good to put at least one on all of them.
  
• Soul Tether (Fortuneteller). This is a guaranteed revive skill that all of the guys can have and it only costs 30 MP to use. It gives Kasuga a more reliable revive and now your entire team is able to bring someone back, no items needed. If you do not have Soul Tether on all of your male party members, consider bringing plenty of revival items.
  
• Endless Desire (Musician). A per turn heal for each character? It doesn't heal for a lot, but it will definitely make a difference in longer battles. On any difficulty below Legend, this can even replace a healer.
  
• Candle Rush (Night Queen). Saeko deserves to have a fire attack.
  
• Evil Begone (Matriarch, DLC). Another ailment-removing skill for Saeko. This one cures allies of sleep, stun, brainwash, and charm.

I'm not 100% certain on how to unlock the True Final Millenium Tower, but I do know that you do not need to complete all substories. I know that it unlocks after you get a message once you complete the Final Millenium Tower, but this only happened to me after the second time I beat it. It may be that you need a complete Sujidex, because I completed it in the time between my first and second runs, but that's just a wild guess.

EDIT: Thanks to JackTrevor and numerous users across other guides, I think it's safe to say you only need a completed Sujidex to unlock the True Final Millenium Tower.

EDIT OF THE PREVIOUS EDIT: Queen Ellie (and a few others) have told me the dungeon unlocked for them without having completed the entire Sujidex. So how does the dungeon unlock? It is a mystery.

[LEGEND]: On subsequent New Game+ playthroughs, once you unlocked the tower it will automatically unlock for you once you beat the game. The message arrives as soon as the one for the notification for the Final Millenium Tower.

The first floor is the only one with an in-game label, all the rest will have actual floor numbers.

Fight 1:
Subjugation-kun x2, weak to electric.

Right off the bat, Saeko should start healing after the Subjugation-kuns attack and everyone else should spam FF/PP. The Subjugation-kuns will be paralyzed for much of the fight. It is probably the easiest fight in the dungeon. If any of your party members die here, you either messed up big time somehow or your team is much too weak for this dungeon.

When one of them reaches half health, you can probably start using Paralysis Prongs to kill one off slightly faster than the other, but they'll die at around the same time either way as one Enforcer will deal as much damage with PP as two other people with FF. Up to you.

[LEGEND]: These guys attack a LOT more often than before, and maybe my agility was just very low for all party members, but the Subjugation-kuns were attacking every other turn, and sometimes twice in a row. They deal close to 1k damage to all my party members at this point, so having Endless Desire up saves them from getting two-shot most of the time. Aside from that, the fight goes the same as before, FF/PP the hell out of them, hope the paralysis saves your team's collective ass, and keep going.

Fight 1:
Mr. Six, weak to blunt.
Grand Slammer, weak to slash and electric.

I would recommend going for Mr. Six, as he's the quickest to kill out of the two and still dishes out some scary damage and buffs if you leave him alone. I found that Zhao's Essence of Ladder Acrobatics killed him in three uses each time I went through. If you target him with all blunt attacks, you shouldn't even have to worry about the Grand Slammer's damage before you can start paralyzing him with FF/PP. Note that the Grand Slammer is not weak to Blunt, so PP actually will deal way less damage than FF in this situation.

[LEGEND]: One of Grand Slammer's attacks here hits two or three times and deals well over 2k damage if you do not block, one-shotting them. Be aware of that.

Fight 2:
Scarredface, weak to slash and water.
Chinese Mafia Officer, weak to guns and electric.

I had Zhao focus Scarredface with Essence of the Pheonix Blade while everyone else focused on the other guy with FF/PP.

Fight 3:
On this floor, you face Saejima Taiga. For some reason, he is weak to electric here, even though the Sujidex lists him as being weak to water.

[LEGEND]: This guy can deal over 1300 damage with Shoulder tackle, so just be prepared for that. That's without his attack boost, by the way. Forgot to block against his Double Backhand and he very nearly sent Kasuga straight into the shadow realm. Keep Peerless Resolve up.

At around 40% health, he goes into a sort of second stage and boosts his attack for the rest of the battle. You should keep an eye on his health because once he reaches this point, his health will not go down until he does his attack boost, so don't waste your MP if you don't see his health going down.

In the last little room by the elevators, there are two suitcases with a Staminan XX in each. Feel free to use two here so these don't go to waste. These actually show up at the end of a lot of floors, so keep an eye out for two suitcases near the end of the floor.

Fight 1:
Flash Mobber, weak to water and fire.
Ironclad Chef, weak to blunt and water.

These guys aren't much of an issue. I would try to target the Ironclad Chef purely because he has less health and will probably go down faster, and also because his attacks are just a little more dangerous (they can burn). I used individual attacks for each of them, as I was caught off-guard by the weakness change, so Ice Breaker to take down Ironclad Chef, and Megaton Throw to take down Flash Mobber.

[LEGEND]: Flash Mobber attacks twice in a row and can two-shot party members, keep an eye out for that. Aside from that, no real danger.

Man-eating Loan Shark, weak to water.
Juggle-O, weak to fire.

The Juggle-O has less health, so spam Megaton Throw at him or use Essence of Phoenix Blade. The loan shark will take a little while to take down, in all difficulties. The only problem here is that this guy can spawn in Yakuza friends to help him out, but they are all around lvl 50 so they aren't worth focusing on. If they do spawn, try to use attacks that hit multiple enemies, or sometimes you can just not bother at all. Usually, they get charmed very easily thanks to Lovedrunk Typhoon and I just leave them for last.

Undertaker, weak to electric.
Scarred Yakuza, weak to slash and water.

NOTE: If you run around the outside of the room (take a right as soon as you enter the room and run as close to the glass as possible) it is possible to skip this fight entirely. You have to run, not walk.

I had Zhao target the Scarred Yakuza with Essence of the Pheonix Blade while everyone else used Fulminating Forecast. The Undertaker is most likely going to die first, he has way less health. Keep in mind that the Scarred Yakuza can call in Yakuza as backup, but just like in the previous fight these are only around lvl 50 as well. Just switch to attacks that hit multiple enemies when they come up.

Forsaken Samurai, weak to electric.
Nameless x2, weak to blunt and electric.

You want to have Kasuga use Peerless Resolve here just in case, as the Forsaken Samurai can easily dish out 1300-1500 damage when he uses Hasty Shotgun, which may happen at any moment, even after the Nameless attack. It certainly caught me off guard and actually killed Kasuga on my first run when I went in and forgot to heal him, thinking I'll be fine based on how the other battles are going.

Aside from that, you can just spam FF/PP on them, or Essence of Ladder Acrobatics. The Nameless usually go down in about 3 uses of these big kid moves.

At the end of this floor, there are another two briefcases with Staminan XX, as well as a recovery point and item box.

Fight 1:
Emperor of Funk, weak to slash, fire, and water.
Urbane Legend, weak to water and electric.

Use Essence of the Phoenix Blade on the Emperor of Funk while the others use FF/PP to take down the Urbane Legend.

Alternatively, you can just spam fire moves at the Emperor of Funk to take him out first then deal with the Urbane Legend alone, but both methods seem to go by just as quickly and I'd prefer to have Urbane Legend paralyzed the whole time.

Fight 2:
Edison of Anarchy, weak to slash and electric.
Tinkersman x2, weak to slash and electric.

Just spam FF/PP, ezpz.

Fight 3:
The last fight here is with Majima Goro and he is weak to fire.

He's incredibly weak, and usually you can forgo healing a few turns to let Saeko have a go at him too. I believe he's one of the weaker bosses, with some of his combos dealing measly 500-ish damage. Just spam Megaton Throw and Essence of the Pheonix Blade on him.

He doesn't have multiple stages like Saejima, nor the damage. You'll be done pretty quickly and enjoying another two free Staminan XXs at the end.

Fight 1:
Dr. Black, weak to blunt and electric.
Invulnera-Billboard, weak to electric.

Spam FF/PP, paralyze the annoying billboard guys while you have someone deal with Dr. Black. His buffs aren't that big of a deal, but he still goes down much quicker than the Invulnera-Billboards so you may as well deal more damage to him first.

[LEGEND]: *cries* They take so long to kill...

Fight 2:
Bohemiarch, weak to blunt, slash, and fire.
Biker Vanguard, weak to slash, water, and electric.
Bedeviler, weak to slash and water.

All of these guys are weak to slash, so my personal preference was to dispatch them using Diehard Skewer. Keep in mind that this attack deals a bit of damage to the person using it as well, but only around 200. The Bohemiarch tended to die first for me, so the rest I took down with either Diehard Skewer or Ice Spreader.

[LEGEND]: Biker Vanguard can deal some nasty damage in a few attacks, although they may have just been crits, but still keep an eye out for that and keep everyone's health topped off.

Fight 3:
Ultimate Champion, weak to slash and water.
Ghillie Man, weak to fire and electric.

I don't have much advice here except to keep everyone's health topped off. Get rid of the Ghillie Man with Essence of the Pheonix Blade, and then whittle away at the Ultimate Champion however you like. I used Ice Spreader to take him down. The guys have the potential to do some high-damage combos, so just be aware.

There are yet another two Staminan XXs waiting at the end by the stairs here.

Fight 1:
Pornogra-Pharoah, weak to electric. (Stop getting on my ass about this it's actually misspelled ingame)
Weltraiser, weak to blunt and electric.

NOTE: You can actually skip this fight too! I managed to do it by running toward the first column that appears to hide from their line of sight, then stuck as close to the glass to my right as possible, and then I was home free. Make sure to keep running. I only managed to do this once, though.

If you do have to fight them however... Spam, my child, spam FF/PP. It is your destiny in this dungeon.

Right after this fight you have a recovery point and item box in the kitchen.

Fight 2:
You final fight on this floor is with Tendo Yosuke and he is weak to... You guessed it, electric!

The Tendo fight here is similar to his usual tower one, in that he does not use any of the one-shotting abilities that were present in your first fight with him in the story (thank god for that). You've probably noticed by now why Fulminating Forecast is a must-have in this dungeon. Well so is Paralysis Prongs, but that's sadly only exclusive to Enforcers.

Make sure Kasuga has Peerless Resolve up for this fight, as Tendo still has a habit of sometimes attacking the same character twice (maybe even thrice but that's never happened to me) in a row and believe me, you won't live through some of his combos if he does them twice. Keep up Kasuga's Fearless Command and Peerless Resolve, make sure Endless Desire is up, and get Saeko to use Lovedrunk Typhoon whenever she isn't healing.

Tendo, like Majima, doesn't have multiple stages, so don't worry about him becoming invulnerable for a time.

Enjoy another two Staminan XXs at the end, and at the start of the next floor there is a recovery point before you go into your next fight, so don't bother recovering before this.

Fight 1:
Your final mini-boss fight before the big boss is with Kiryu-never-killed-a-man-Kazuma, and he is weak to water.

NOTE: Going up against Kiryu, it is actually recommended that you have Saeko and possibly even Eri in your squad, as he is apparently programmed not to attack female party members (fun fact, according to the wiki). This is the case when you fight him the first time in the story and it stays true in the tower, and I can confirm that he has never attacked any of my female party members. What a chad.

I'm not sure if his phases here are meant to be different or maybe I had a bug, but in the two times I fought him in this dungeon, his invulnerability phases when he changes stances occurred at different points in his health. Keep an eye out on his health to see whether or not you're actually dealing damage to him.

On my third run where I fought him, I managed to see when he entered his invulnerable phases; he enters Rush style at around 75% health, Beast Style at around 40% health, and then Dragon of Dojima Style at around 15% health. The bug(?) I mentioned above may still apply, though.

Only real thing worth noting is that in Rush Style he counters a LOT, so try to keep using abilities and don't resort to normal moves.

Keep in mind that if you somehow get him to his next stage before he attacks, he will go through a normal attack first before switching stances. When he does this, his invulnerability is still up- he needs to actually switch stances for it to go away.

Use Ice Spreader, make sure Kasuga's Fearless Command and Saeko's Lovedrunk Typhoon are up as much as possible. If you can, keep Endless Desire going as well to reduce the time you spend making Saeko heal. Keep in mind that when Kiryu changes phases, he removes Saeko's defense reduction from Lovedrunk Typhoon, so recast whenever you feel the need to.

[LEGEND]: Not entirely sure if he attacks more often here or not, might have just been my bad luck.

Don't use any items to heal, just head back to the area just before this fight to use the recovery station! This is the room just before the boss, so make sure you're ready! There are also two more Staminan XXs at the end here!

Your last fight in the tower is with Amon.

Note: @An Actual Skeleton brought up some useful information, if you wish to go this route:
"It's not pointed out in any way here, but you should know regardless.
Poison works miracles on the "Entities" minibosses, especially Tendo (I'm talking 250 clean damage at each of their turns). Have at least one lady in the party with Trickster hat, Joong with his poison shot and someone else with the Devil Rocker Intoxication debuff. Poison shot works on Amon as well, it wont debugg him but its still a good amount of damage."

First off: Lovedrunk Typhoon, Endless Desire, Peerless Resolve and then later Fearless Command. Keep all of these up as much as you can, especially Endless Desire and Kasuga's Peerless Resolve.

Like before, Amon has stages similar to Kiryu's in which he changes stances. A majority of the fight will go off without a hitch and you'll start feeling uneasy that it's going too well, and that's because the fight is insanely easy until you start hitting the lower half of his health.

He switches to Rush Style at around 75% health, just like Kiryu. During this stage he will counter your normal attacks, so try not to use those.

At around 40%, Amon uses Guard Order. At this point you want to call up Mr. Masochist to give you his damage reduction and get all of your buffs ready because this stage is where ♥♥♥♥ hits the fan. Right off the bat, he spawns in Kiryu, Saejima, and Majima to fight with him. Now you have three minibosses to worry about on top of Amon and not just that, but they are a higher level than before too. These are not just grunts with less health, they are tougher than before, don't let your guard down and keep Saeko healing as much as possible. Unfortunately, they have multiple phases just like before, with the exception of Majima.

During this stage, Amon will use Malicious Grin, which makes your party cower in fear while raising his attack, and it has a very high chance of proccing on your party. Thankfully, this is why I kept the Bracelet of the Four Gods on Saeko- she isn't affected and thus can keep healing until the ailment either goes away or there is a free moment to remove it.

You need to take them all down one by one, do not focus on all of them as that can potentially kill your entire team. At low health, these bosses will use their specialty summon move on your party. Amon uses Essence of Orbital Strike on you, which can easily wipe your team without buffs. Even with buffs, it left my whole team at around 200-300 health.

NOTE: You can avoid it by killing them off as quickly as possible, but there's no surefire way of doing so. I had a few battles where I managed to kill Saejima and Majima without them activating their summon moves, but since Kiryu and Amon have their invulnerability stages that provide a safe buffer for them to activate these, I don't think it's possible to avoid theirs. Be wary of this as you whittle away at their health.

Again like Kiryu, his last stage comes up at around 15% health, but there isn't really much to mention about it except that you should be topped off on health as much as possible.

It's best to finish them in order by health, starting with Amon (he will have the lowest health). Take him out and fully heal from the Orbital Strike, then take out Majima while recovering from his heat move. Then you take out Saejima and take some time to recover from his move, and then finally you can take down Kiryu, as he either has the most health or the water attacks simply aren't as strong as the rest of your party's moves, because he will take a while just like before. Remember that Kiryu's summon move has a chance to instantly knock someone out- I'm not sure if this applies to your team but it's best to assume it does. Take him out, and you're done!

[LEGEND]: Amon's Essence of Orbital Strike one shot my entire party on this difficulty. On my second try, he one shot everyone except Adachi. Keep Peerless Resolve up, can't stress that enough.

While I don't advise doing this, I couldn't really ignore it as a possibility. There's a reason this is at the end and not the beginning. Now this tower is no joke. It's really tough, having to go through all the enemies. If you don't think you have a big enough ♥♥♥♥ to go through it the proper way, you could go through it the way I potentially could have. That is, overload your computer so the game fails to load enemies properly. I did try this on a second attempt and managed to skip over a majority of the fights, including all of the miniboss fights. Of course, this doesn't work for everyone. But if you're one of the people whose game has trouble loading, that might be the option for you if you want to risk having your game and computer crash. I really don't recommend it of course, for many reasons.

Hopefully, this helps some people. The dungeon is difficult, but much less so when you're fully prepared. When you're done, you are granted the achievement; Victory of the Millenium.

If there are any problems or questions, please let me know. I've done the dungeon a few times now, this is just the most reliable team I've done it with. I probably know a thing or two. If I think of anything else later, I'll try to add it here.

Congrats to all who've gotten it, and good luck to those attempting to get it.

December 6th:

• Fixed some spelling/grammar errors.
  
• Added a few Legend Difficulty tips.
  
• Fulminating Forecast and Paralysis Prongs are referred to now in tandem as FF/PP.

December 11th:

• Added the "best weapons" list I used as a reference in case someone else wants to know what they are, in the Tips section.

December 14th:

• Fixed up grammar, changed up the wording. Gotta keep things spicy, ya know?
  
• Finally finished adding Legend difficulty information.
  
• Changed the info on how to unlock the dungeon, thanks, JackTrevor!
  
• Reverted the info on how to unlock the dungeon, thanks, Queen Ellie!
  
• Changed "♥♥♥♥ hits the can" to "♥♥♥♥ hits the fan" as who the hell writes it that first way? Was I high?

December 16th:

• Fixed the title. Didn't see it misspelled "millennium" this whole time.

December 18th:

• Clarified that everything in this guide is advice, not a rule.

I've beaten the dungeon on the hardest difficulty and finally added the information to the guide, so all other edits past this point are for aesthetics/grammar/fixing purposes only.