At this point you finished the game and at least beat the True Millennium tower. Before taking on the beast, it's best to be prepared . It's recommended that you're party of four are lvl 99 and job lvl 90. For the party I recommend Adachi, Saeko and Zhao. Next would be duplicates of job accessories,it'll make the grind so much easier.
Base XP 1.0
One job accessory XP1.5
Two XP 2.0
You should stop by at the pawn shop and pick up some smoke bombs. This battle item will allow you to leave the battle. Just in case you can't escape. Note cant be used in the tower. Its for the XP grind. Also stop by at one of the vending machine and buy apple defense. Not necessary but it helps having everyone have max bonds. Just give flowers to saeko and just hit the hostess club with the boys. Gives more access to more jobs. Plus bring Premium med kits and Resurrection Bolus
Also I made a awful job accessories guide.(that was the first guide i made btw)
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2325464202

The recommend level is 90 but I went for 99 for the main jobs.
Which are.
Enforcer for adachi
Idol for saeko
Hero for Ichi
Gangster for Zhao.
You don't need to level all the jobs to 90.
But it's best to have most jobs at least level 30. I highly recommend having Fortuneteller,Foreman;devil Rocker,host and this optional Bodyguard just for the extra attack. Hostess, Nightqueen and Matriarch. The more jobs that are level 30 helps.
The skills you'll need are.
Fulminating forecast bonus soul tether
Megaton throw
Alats hold
ice spreader
Candle Fire
Evil Begone

Head over to the sewers and reach the second floor. You either can do the fights or just run away. Our main target is vagabonds.
These bad boys drop about 400k exp. Their defense is pretty high and the best way to kill them would be a critical damage. I recommend saeko chips skills since it has a high chance to land a critical hit. They have a chance of fleeing the battle. At this point escape or finish the fight. There are only 4 vagabonds that spawn. So head back where you came from and reenter the area. As it'll re-spawn them, instead of leaving the dungeon. Rinse and repeat.Hopefully you'll have duplicates of each job exp item. You'll want that 2.0xp boost. Have your party aim for level 30 for each job after that change your jobs

Here's the recommended gear.
Note I only maxed out ichi bat.
Ichi
Legendary heros bat EX EX
Quixotic helm
Quixotic armor
Quixotic greaves
Brawlers God mouthguard
Eye of dragon
Adichi
special ops riot shield EX+4
Cyber helmet
Dragon kinagashi
Illegal boost
2 war gods keystone
Sakeo
Celestial Microphone EX+4
Cyber helmet
Dragon kinagashi
Dragon boots
Bracelet of the four gods
Military insoles
Zhao
Curved crystal Saber EX+9
Dragonhelm
Dragon kinagashi
Dragon boots
Kiwami mouth-watering
Diabolic wristband

FF=Fulminating forecast
USS-Ultimate stack slap
PR-Peerless Resolve
MT-Megaton Throw
IS-Ice spreader
AH-Atlas hold
FC-Fearless command
EBG-Evil Begone
LDT-Love drunk typhoon \
CF-Candle Fire
EOPB-Essence of Phoenix Blade
Peerless command-PC

Hopefully things go well.
With each encounter start by applying buffs such has PR,PC and LDT.
Then Spam Abilities and heal when needed
Things will play out the same.

IF no one needs to heal. Use LDT and PR,Spam FF and if you can use PP. Can cause rage and self attack buff. If possible also use PC
Use PR on ichi turn, if you can use LDT and PC. I recommend using PP on him and and as fast possible. Removing him makes it easier. He can apply debuffs and buffs.
Spam FF and PP. Can apply a defense debuff. IF its Zhao turn use FF on the slammer.
Hopefully that cause him to be paralysis
Apply PR.LDT and PC when possible Cast IS and use zhao basic attacks also try EOPB. if your party is charm use EBG
Can apply poison and charm.
Spam FF and PP
He's weak to ice but doesn't feel like it.
Try using different abilities. PP and USS help.

Use PR,LDT and if you use PC. Spam pp on him and Ultimate fat slack.
Can cause burn.
Use Essence of the Phoenix blade and spam Megaton throw and if you can CF.
Can cause fear and confusion and can cause party to become susceptible to status ailments.
Use PR,LDT and if you can use PC. Spam essence phoenix and megaton throw. Use ichi buff fearless command.
Can cause burn?
This clown is annoying and is only weak to ice.
Ultimate stack helps and use PP.
Can cause charm
Next encounter you can run by. Just hug the wall on you're right.
Use PR,LDT and if you can use PC.This encounter is pretty easy. Just be careful and play smart. Spam FF and if you can use PP. If you're lucky they might never have a chance to attack.
Can cause silence.
Be Careful his shotgun can cause damage. Lucky he's also weak to FF.
Cause burn?

Use PR,PC and LDT. Heal when needed. Same always spam FF and if you have IC with saeko.
Can cause drunk

Spam essence of Phoenix or MT,IS
Debuffs accuracy

Use PR,PC and LDT. Heal when needed. Just spam FF. Their all weak to electric.
Can cause burn.cold and paralyses. personally i never bothered to cleanse the affects.
This fight is pretty easy. Spam EOPB,CF and MT.
Use PR,PC and LDT.
Can cause paralyses

Apply PC,PR and LDT. Spam FF
Also weak to FF, spam that. Can hit very hard.
Can apply debuffs and buffs,

Use PR,PR and LDT. Spam essence phoenix and if you can use fat stack.
Can heal, debuff attack.
♥♥♥♥ this encounter. Be very careful. Use peerless resolve. The biker vanguard hits stupid hard.
Spam EOPB and IS.
Can apply cold and has a small chance of one shot party member's.
Watch out dude can hit stupid hard.
Spam FF and if you can vampirism.
Use PR, PC and LDT.
Be careful when he's muscles are ripping.
He'll use a rising corkscrew. Which hits insanely hard. Spam EOPB and IS
Spam MT and FF plus USS does pretty good damage to him.

Use PR,PC and LDT
Spam FF
Can apply debuffs and buffs.
He's also weak to electricity so spam FF and PP.
Hits hard and can apply cold?
Spam FF and PP, PC,LDT,PR. This fight might take awhile. If no one needs to heal block
with saeko

He doesn't hit any women. So saeko will be completely safe. Spam IS and use buffs. At some point he won't take damage. Just block since he's just changing his stance. Then you can hit him with the ice. Beat him and go back restore HP and MP.
This is it. The good thing is if you die. You can retry the fight.
First thing apply buffs. LDT,PR,PC. attack normally.
Just spam any of you're ultimate ability. Since he can't dodge or hit you back. PP helps but there's a chance you can miss. Personally I had saeko block.
Spam AH it does good damage plus there's a chance your defense will boost.

I recommend taking down amon first. After he's health goes below the A. Refer to the picture Call Mr masochist. He helps a lot. He'll still be in beast mode. So spam AH. Be very careful at some point he'll change to his last stance. If you can give ichi some apple defense.
He'll start with fear. Lucky sakeo won't be affected and if you are more lucky maybe others won't be. It's best that you heal. Hopefully you'll be able to tank the hit of the laser. With the help of AD PR and Mr M. Heal and spam ultimate abilities since its all his stances.
I recommend going for majima next. Personally he's the easiest.
Spam EOPB and MT. But this time it's different when he's health gets lower he'll use poundmates ability. so be careful.
He's weak to ice but doesn't feel like it. Use PP and USS. when he uses tiger essence. Which will give him a huge attack boost. It's best to play safe and have everyone guard.With some turns the buff will end. Heal when needed. Like with majima he'll use the pound mate ability. Which lowers your defense.
Same has before spam ice. Once you start to lower his. Play it safe itchi have him block and apply AD. Kiryu will do the essence of the dragon God. Kill him and then GG.