This is available after getting Archangel Flonne.
After this achievement, there will be a new bill in the Senate, The Legendary Delinquent Appears!
. Pass that bill and the post-game chapter will open.
This stage consists of three sections - a No Ranged section that you start in, a Magic Range +1 section separated by an invincible No Entry, and an empty section with Wood Golems. Make a tower of four people with your strongest at the top, and throw them to the aqua No Ranged geo panel. Have your strongest throw to the right, leaving room for you to stand by the sewer. On the next turn, throw that geo panel into the green section. Clean up the wood golems and nekomata, then use your own ranged characters to pick off the ghosts and Raspberyl.
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. However, you should really consider opening up Mt. Ordeals and reaching the final stage, as the next few levels will be incredibly difficult without it.