Crypt of the NecroDancer

This is by no means a complete "here you go" solution, nor will it ever be. This will only be obtained by those with enough skill and dedication to do so. There is no easy way through it.

So first off, to unlock all characters mode, you must first beat the game with every character besides coda, whom you unlock by completing an all chars run. This means you must first beat the game with Bard, Cadence, Melody, Eli, Dove, Dorian, Monk, Bolt, and Aria. Once done, you will find the all chars stairway in the character select stairway.

In completing the game with each character, you will learn their subtle nuances, as well as the ins and outs of the game, and what is required of you here.

Now, for the actual all chars run, you have to beat all 9 characters in a row without dying. If you do happen to die or restart after completing a character, your progress will be reset and their staircase will be unlocked again. The only thing you have going for you is that you get to choose the order of characters you play. This means that in the end, it's all up to your preference. However, I would strongly advise starting with your weakest characters, and working your way to the easiest, or your most consistent.

Generally, your all chars layout should go something like this:
(Aria, monk, bolt)-(Dorian, dove, Eli)-(melody, cadence, bard).

The sets of brackets are basically hard chars, medium chars, and easy chars. The characters within the brackets can be interchanged depending on your preference. For example, I initially intended to do it in the exact order above. However, with all the practice I've put in on aria, I feel like I'm more consistent with her than monk and bolt, so for me it'll probably start with monk and bolt, with aria taking third spot, then Dorian, dove, Eli, etc.

Now it goes without saying that you should practice the bosses extensively, especially with characters you struggle with, and the final bosses for aria, melody, and cadence. If you're attempting this run, you should be pretty well versed in most of them anyways, but monk, bolt, Dorian, and Eli could use some different tactics, monk you have to leave a path through gold or save bombs to clear a way out, bolts speed can make otherwise trivial boss fights a pain, Dorian's double square jump messes with typical strategy, and Eli you must plan between kicking bombs and planting bombs since he has no regular attack. The rest should be fairly consistent for boss fights.

For some tips on most bosses, check out my flawless victory!(Aria low%) solution here:
Please note that these are no item strats, and depending on your equipment it can make these fights even easier. Also, those tips are in arias zone order, so they're backwards. For example, Coral riff 4 is Coral riff 1 for everyone else. Coral riff 3, is Coral riff 2 for everyone else, etc etc.

For additional strats you could try checking out IllegallySam's aria guide here:

Bard:
This is the easiest character. You dont have to follow the beat, so you can take all the time you need to assess the situation and figure out how to handle it. He is also great for practicing enemy patterns in general before moving on to a regular paced, or even double paced character. I believe he only starts with 2 health though, so you do have to play carefully in the beginning.

Cadence:
We'll call her the default character since she plays normal and has no gimmicks. She will teach you how to play the game as intended, since any missed beat causes you to lose your coin multiplier, whilst also allowing the enemies to advance. Furthermore, you'll learn to move quickly to clear out the boss to open the next door before the song ends and you get dropped into a dangerous room to start the next floor. You also have 2 new bosses to contend with. Dead ringer, and the necrodancer. The former isnt too bad, but you can get screwed on the necrodancer really fast if you neglect one character or the other. I found it best to just rush him most of the time so he doesnt get much chance to attack, but sometimes the enemies dont allow it.

Melody:
Melody plays basically the same as Cadence, except you are stuck with the lute as your weapon. Overall its actually pretty decent other than for a few enemies or situations where it can make it a little tougher. Other than that she has a different version of necrodancer to contend with. This version is basically all about rushing him and knocking him into the lava pit at the end before the walls close in and crush you.

Eli:
Elis gimmick is that he gets no weapons, but has unlimited bombs. You can also kick his bombs forward 3 squares after planting them. Overall, once you get used to this, hes a pretty easy character. Only a few enemies should cause any problems, such as enemies that move every turn, and maybe the sphinx things on stage 4 that jump 3 squares at once.

Dove:
What makes dove tricky is shes stuck with flowers as a weapon that confuse enemies, but does no damage. This mostly makes it a rush to the exit. Sometimes you end up having to lure a bunch of enemies out of a room to be able to make it through. Other times you can dig a path around.
Hitting enemies with the flowers still makes them drop gold, but she only has a few places she can use it, such as the random item shrine.
The plus side is she starts with a titanium shovel, the floor exits are already open, and you dont have to fight the main bosses. You also get one shop item for free from every shop.
Dont bother trying to switch her ring. Only 1 or 2 others would be useful, but her starting ring gives her 1 heart and makes floors slightly easier, so its easily one of the best for her run.

Dorian:
Dorian starts with extra defense and attack, but hes stuck with the cursed boots which are atrocious. They force you to jump 2 squares every turn, because every square you jump with them turned off causes you to take damage. This totally obliterates typical strategy. Half the time it isnt even worth going for shops or portals, cuz they will usually force you to take at least one damage to get the item you want and get back out.
For many enemies you may be forced to use wall hits to buffer beats and lure enemies close enough to attack. Luckily he starts with the pickaxe, so you can get multiple buffers from the heavier walls. You will also be forced more often than not to dig extra spaces in walls so you can be in proper position to land on items and exits with his 2 square jump.
Its been a while since ive played, so i cant get in to boss strategies with him right now, but 2 square jump makes them a fair bit harder unless you managed to snag a bow or crossbow.

Monk:
So, monk is a complete pain in the ass. He starts with 2 health, and any gold you touch is an instakill. This means you must completely rework your muscle memory and how to play the game. Know your weapon. The sword will slice 2 squares right through the first enemy, where pushing forward twice would kill you, while the spear, despite having 2 square range, only hits the first enemy, so pushing forward twice would kill both enemies, and a third time would kill you. This subtle change can be the difference between life or death. I prefer the spear myself, so i can focus on one enemy at a time. Bow or crossbow is my real preference.
Once you rework your muscles not to pick up gold, he plays pretty much normal, but you have to remember to lure enemies out of tight spaces to leave yourself a path through the gold. This is also important when planning your strategy against the bosses. Most notably deep blues. Its good to save a bomb to get through there just in case. The other thing to note, is while you cant pick up gold, any gold laying around when you complete a level is added to your total. He has few places to use it, but it can come in handy if you find a random item shrine, or a purchase or transmogrify portal.

Bolt:
The only thing that sets her apart is that her beat runs at double speed. And that is enough. It is extremely hard to keep the beat for the whole song for anything past the first world, or even halfway through the second world on a controller. Thankfully she starts with a spear, so you get to keep a little range from the enemies when attacking. If you can switch to a bow or crossbow for even more range, i suggest you do it. You're going to want as much range as possible between you and the enemies for all the missed beats youll probably end up taking.

Aria:
Easily the hardest character aside from coda until you get loads of practice with her. Then dorian, monk, and bolt might be bigger threats. Refer to the links i posted for mine and illegally sams aria guides for fairly in depth info on her.

Still a work in progress, If i can, id like to try to add a few boss strategies in in the future.