what is the best strategy to unlock the carnage dimension

[ManoL]

Well, I passed the third part of the post game and I'm training to go to the end of the post game, but even so I thought I could ask for tips
like which characters are going to be useful, skill tips and good set up these things, any tip will be useful thank you

[Pickleton]

Before unlocking Carnage Dimension:

Level a character to lvl9999 by doing Martial Trial Final over and over.
My suggestion: Pram and Metallia (magichange Metallia onto Pram)
While doing that, make sure to subclass Shroom on one of them at least far enough for Eryngi Fluid evility. The rest of the subclassing doesn't matter so much, but mastered Prinny gives good stats for when you do finally reincarnate.

Reasoning:
1) Pram and Metallia are crucial characters for the 1-turn Double Fake extract farming strat.
2) A lvl9999 character with Eryngi Fluid (from Shroom) evility that gets hit will 30% give you a Mushroom Soup (rank39 SP healing item). If you're on a carnage stage at that time, it will be a carnage version. If you go into carnage Mushroom Soup item world and abuse Filler item, you can cheese the bonus gauge to get high rank Carnage Items instead of basically starting the game over at rank1 (use Fillers, step on gate to go to next floor, repeat).

After unlocking Carnage Dimension:

Your first main goal after unlocking the Carnage Dimension is to hit 20mil base stats on your characters. Equipment is extremely secondary.
For those 20mil stats, you can get 10mil base stats from point allocation and leveling up (need to master basically all subclasses), and 10mil base stats from extracts/shards. Subclasses need to come first, at least for your first several characters. This is because farming extracts efficiently requires you to already have at least 10mil stats and a specific evility setup (link below).

There are probably a dozen good strats for subclass farming as well, but my preferred method on that is as follows:
1) Turn as many of the characters you're subclass grinding into monsters.
2) Give a character the Trash Pickup evility (Raspberyl).
3) Put anyone in the Supply Depot squad
4) Give as many characters as possible (up to 20) the Prinny Instructor (Valvatorez) and Prinny Hat (F.u.k.a.) evilities, and put them all in the Foot Soldier squad. Of course this 20 should include the characters you're subclass grinding.
5) Set cheat shop enemy strength to 20 star.
6) Go into Item World of a R39 carnage version item (the Mushroom Soup works).
7) Create a tower out of all of the characters you're subclass farming, magichanging the monsters to the humanoids to allow them to stack.
8) Attack a Geo Symbol, it should Deathblow because of the Trash Pickup evility.
9) Watch as you max out a subclass for every single character on the tower.
10) Use a Mr. Gency from the base panel (Support Squad).

Now that you're sitting on 10mil base stats, you can start farming out the 10mil stats from extracts.
I'm not sure if there's another better streamlined video out there, but refer to this guide for for evility selection and party setup.