This can only be obtained after beating the main story.
After beating the game, there will be a new bill in the Senate for the Dark Hero Live Show. Pass that bill and the post-game chapter will open. The stage itself isn't anything special; levels are on par with the final chapter you just cleared, and the enemies consist of 2 sections of 3x4 enemies, 6 bouncers in a line, and Axel. Use your preferred AoE attacks to thin the crowd and Valvatorez with a Tyrant Flughude to clear all but one of the bouncers. Axel is unique in that he takes half damage from humanoid characters, so if you haven't trained monsters outside of Desco up to snuff, he's tougher than he looks.
If you're still struggling, you need to power level some more. At this stage, your best options are 5-2 or 9-4, as you can easily abuse the level 99 glitch (a level 99 enemy gives XP and mana of a level 323 enemy) to power level.
5-2 is for units that can hit at least a 2x2 ranged area. Set the enemy level in the cheat shop to 8 to make them all level 99. First, go to the bottom right of the screen and move the yellow geo block onto the rest and destroy it to make the entire area EXP +150%. This is an excellent area to level Valvatorez with Tyrant Flughude, but the cockatrices have a silence aura, so you'll need someone ranged there. If you magichange a monster into a weapon, the EXP is copied, not split, to the monster as well. Stats don't matter, so a level 1 monster will work fine.
9-4 is strictly for monsters and Desco. Set the enemy level in the cheat shop to 1 to make them all level 99. Use Valvatorez and Fenrich to throw the geo panels to give EXP and Mana +50%. Move Desco up and any monster at all right behind her. Use that monster to fuse and make Giant Desco. The main advantage is that Yog Sothoth now hits a 5x3 area, meaning Desco can one-shot the entire group. If she's struggling with power, her passive boosts her ATK with adjacent allies, and Male Fighters have the ability Fight Shot to double ATK for 3 turns.