There's a lot of misinformation around this achievement on the Internet.
You don't need all or most jobs maxed out like some guides say, that would require an insane amount of grinding, it was tedious enough to max out one job for each character. Would it be easier if you do max out more jobs? Sure, but not really worth the effort in my opinion, I beat this dungeon with only one job maxed out for each character and some other jobs at level 30-50 (Fortune teller, Foreman and Host are important to level up, each of those jobs has one different elemental universal skill that will be very useful, especially fulminating forecast since 80% of the enemies in that dungeon are weak to electric damage)
IMPORTANT - Get 10 miracle kimchi from the kimchi vendor (Unlocked after completing "The Miracle of Spice" substory.) These are incredibly helpful for the Amon fight, I'll explain why later. Also, stack on as many recovery items as possible, you'll need them to recover your party's HP between the battles inside the dungeon.
I had my party as Ichi (Hero), Saeko (Idol), Joon-Gi Han (Hitman) and Zhao (Gangster), you really dont need anyone else, Adachi is too slow, Nanba dies too fast and Eri, well, Zhao and Han outDPS her anyway, and Saeko is just a better healer, so no reason to run Eri as idol either.
Regarding the weapons, gear, and the stats of the characters that I used:
Ichiban:
Han:
Zhao:
Saeko:
PS: If you want to farm money for the EX weapons, there's a method that can give you about 75 MILLION per run in the regular final millennium tower dungeon. Here:
When you enter the dungeon, it's important to note:
YOU CAN RUN AWAY FROM THE FIRST FIGHT. Many people say you need a base xbox or whatever to run past it (cause the enemies wont load in time), but actually, you can just run away (holding LB when the fight starts) from that fight, so do it.
Another thing to note: You can avoid the second fight on the third floor by hugging the left wall behind the dude with a pink suit (won't work everytime though, at least in my experience, but its worth a try), and you can also skip the third fight on the same floor by hugging the right wall
The entire dungeon until Amon is pretty much abusing the enemy weaknesses and healing with saeko (or lowering the enemy defenses if everyone is already at full hp, attacking with her it's not worth it tbh), this guide here says what every enemy is weak to, very helpful:
https://gamefaqs.gamespot.com/ps4/272324-yakuza-like-a-drago...Amon fight: Start using the Miracle Kimchi on every party member except Saeko, that'll make them have turns more frequently and is VERY helpful. Spam him with Phantom Dive (Han), Phoenix Blade (Zhao) and Bonecrushing Bat (Ichiban), for Saeko, just restore their MP if there's no need to heal or user another Kimchi on Han, Ichi or Zhao if needed.
When Amon stops taking damage, just restore your MP, use debuffs or items, for Ichiban, use PEERLESS RESOLVE (That'll save you from a attack that would kill him otherwise). When he changes his stance, he'll be vulnerable again.
When he changes his stance to Beast Style, that's when things can go south. Summon Mr Masoquist poundmate when you get the chance. When he summons Saejima, Majima and Kiryu, he'll probably use Orbital Laser in the next turn, so make sure everyone is healed up and Mr Masoquist effect is active.
I focused only on Amon after he summoned the Tojo trio, Amon deals way more damage than them and is a much dangerous threat, not to mention he can also use a move that can instakill every member of your party (Effigy of Demise), that got me killed like two times. So just focus on him with Han and Zhao, have Saeko heal everyone up and Ichiban using Orbital Laser. When he's defeated, the rest of the fight is kinda easy. Focus on Majima because he can paralyze you, he's weak to fire. Saejima comes next, he's weak to electricity, Kiryu is weak to water. (WARNING, they all can use their respective Special Attacks from their poundmate summons when their health is low, so be careful, I almost got the whole party killed because I tried to play around with Kiryu when his health was low). Video of me completing it for reference:
By the way, everytime you die to Amon, you can pay half your money to restart the fight with full HP, you don't need to restart the whole dungeon (only if you die before getting to him)
English isnt my mother language, so sorry if this guide wasn't clear enough.
Good luck to anyone trying this.