Heyo. Here I will list explanations and tips on how to conquer this mode, due to the request of a couple people.
This will be formatted very closely to how I write things in my main 5 Death Wish guides.
This is also under construction since I cannot save this like I can in my guides.
If you spy anything to tweak, have your own tips, or just wanna show appreciation, feel free to comment below!

You will be fighting every boss in their powered up EX states at once!
To enter, you will need 88 stamps and have beaten "Seal the Deal" (You'll need it!)

• Enemies drop less health pons (25-50% compared to Vanilla)
  
• Enemies have their EX moveset
  
• Trying to jump to the 5 tiny platforms where the bosses wait will insta-kill you

• COMMIT MYUUUUURDURRRRR
  
• Defeat the bosses in order of chapter appearance
  
• No hats!

There are 4 main phases, with a 5th secret one if you save MU for last.

• Phase 1 - Mafia Boss
  
• Phase 2 - Mafia Boss + DJ + Conductor
  
• Phase 3 - Mafia Boss + DJ + Conductor + Toilet + Snatcher
  
• Phase 4 - Mafia Boss + DJ + Conductor + Toilet + Snatcher + Mustache Girl
  
• Secret 5 - Mustache Girl's Final Phase w/o the raining Time Pieces

While this is every boss at once, JABrules has gone the extra mile over the course of his mods and given some of them adjusted movesets. Here I will list each boss, their health across the 5 Phases (p1/p2/p3/p4/p5), and any moves that are different from their regular EX versions.

Mafia Boss EX - HP (4/3/3/4/0)

• NO MAFIA BALL, THANK GOD
  
• Unlike when he was 2D, his "Spin Dash" will now send him bouncing across the map, lasting about 3-4 times in Phase 1 and 5+ for the rest of the fight. It will also spawn spark orbs as he rotates after Phase 1. The punish window is dependent on how far away you are from him.
  
• His Ultra/Megacharge now emits a cone who's center is your current location. The punish window is dependent on how far away you are from him.
  
• His whistle move now rains sandbags in a giant circle around you, with the goons going after your current location. He will still jump to your current location when he's done.
  
• His move where he jumped to the background, threw his weapons, and landed on your location is largely the same, but now his weapons orbit around him and slowly come to him. This starts out as 4 weapons, but becomes a dozen by Phase 4. Beware, as they really do Orbit him. As he moves, so do they, so watch out for any weapon getting within your reach. If you double jump just before he lands on you,, you'll avoid damage and can easily Homing Attack him to cancel out the moves. This move has about a 2-3 second punish window.

DJ Grooves EX & Conductor- HP (0/5/5/5/0) each
*As far as I can see, DJ and Conductor do not share a health pool

• NO CAR MOVE, THANK GOD
  
• Their Disco Ball and Knives never spawn clones
  
• They have a new knife move where they will yell a taunt then proceed to spawn a bunch of knives around them then quickly throw them. The punishment window for this is narrow. To dodge, I'd advise jumping as soon as you can.
  
• Their sawblade throwing move from Phase 4 of their fight is incorporated here, they'll throw themselves to one corner, you'll hear the knife SFX, then proceed to throw sawblades 1-3 times depending on the pattern. Best way to dodge this is just jump around.
  
• Their Belly Flop move extends from 2 in phase 2, to 4 times before being vulnerable by the 4th Phase. It should also be noted that their Belly Flop move can damage the other bosses, so watch for that when attempting to hit a boss with them Belly Flopping.

Toilet - HP (0/0/2/3/0)

Standard toilet fight, but no Super Bombs or cages to deal with. The best way I would deal with Toilet is by waiting for him to slam into the ground 2-3 times and expose himself by turning blue. The fruit is very risky as it slows you down.

Snatcher EX - HP (0/0/1/4/0)

His Whack-A-Mole move is from his Ultra Snatcher counterpart, that is to say it is only sawblades that go to your current location.

Mustache Girl EX - HP (0/0/0/10/x)
*MU's Health in "Secret 5" is whatever health she had remaining in Phase 4.

MU in Phase 4 is the same as how she acts in the "Fruit Throw" part of her fight, minus the shield and fruit. If she's the only one left standing, she will take on her Final Phase moves, minus the falling Time Pieces.

An important thing to note is that they still spawn Health Orbs upon being hit. Seems to be "every other hit" from what I can tell, but perhaps a bit RNG?

If you do not care about Bonus 2 "No Hats," then Time Stop + Fast Hatter is an absolute must of a combo. Also be sure you have Magnet equipped. No Bonk isnt really useful but can make recovering from almost falling off the map a bit easier.

Stay calm! Don't get greedy! As nice as it would be to slam down the bosses into the dirt ASAP, you cannot risk getting hit too much! Snatcher is likely to get you the most as his moves are very fast and you'll be focusing on the others!

I find myself jumping, staying in the air as much as possible, for this mission. Most of the moves and obstacles you will be up against are only on the ground, so this is really helpful inn keeping out of harms way. it also allows you to very easily use Homing Attacks whenever you want. Homing Attacks are an incredibly powerful tool, allowing you to hit from a distance, travel the map, AND give you a boatload of i-frames.

A majorly helpful tool to help avoid attacks is to jump off the arena. Falling off will spawn you where you began the level, possibly allowing you to avoid a sticky situation (I find it very helpful in avoiding Snatcher's Red Circle Explosions, any falling down projectiles, as well as MU's Time Piece Barrages). Just be careful that where you'll spawn isnt where the attacks are already landing!

Of course, another great word of advice is DO NOT TUNNEL VISION. You generally want to attack everyone equally (maybe not Snatcher tho) so that they can give you a steady supply of Health Pons as well as make it so bosses die faster, giving you less obstacles to worry about.

If you do not go for Bonus 1, my recommended kill order would be:

• (1) DJ + Conductor - They take up a lot of space with their moves, the sawblades being the real pain in the ♥♥♥. Thankfully though they also are REALLY open to Homing Attacks so I find myself killing them very easily.
  
  
• (2) Toilet - He has very little health but can cause issues with his shockwaves.
  
  
• (3) Mustache Girl - She's not so bad on her own.
  
  
• (4) Mafia Boss - This guy is too busy screwing around moving every which way, making it rather hard to land hits on him. Thankfully, his moves arent really that terrible to avoid, so I think it is good to save him for nearly-last
  
  
• (5) Snatcher - Easily the biggest thorn in my side for this mission seeing as his moves LITTER the floor. But you do not want to go for him first seeing as the OTHER bosses moves make it very hard to land hits on Snatcher.

Defeat in order of Chapter Appearance: This means you kill them in the order of:

• Mafia Boss
  
• DJ + Conductor (interchangeable)
  
• Toilet + Snatcher (interchangeable)
  
• Mustache Girl

All I can really recommend for this is reaffirm my General Advice above! The absolute most important one being DO. NOT. TUNNEL VISION! Hit them as equally as you can, especially to heal, but try to focus more on one until the others are dead. Honestly, this objective isnt terrible once you defeat Mafia Boss seeing as he's the only one that actively runs away while the rest like to get in your face. Tme Stop Hat will be useful here.

No Hats! My kill order listed in "General Advice" will help a lot here, otherwise this really boils down to being attentive and having quick reflexes.

https://youtu.be/lz7KHh_UmGM