This is available after getting Raspberyl.
After this achievement, there will be a new bill in the Senate, The Nice-Bodied Appears!?
. Pass that bill and the post-game chapter will open.
This appears to be a straight-forward stage, and it is, just the initial strategies won't work. Every Prinny on the map has the Heavy Stance evility, so you can't pick them up. While you can throw your own Prinnies next to them to explode them, you're limiting how many characters you can deploy yourself. Additionally, there is an Enemy Boost+50% on the field. However, you'll just have to deal with it - the other section has No Entry and Invincible, and by destroying it, the map becomes unbeatable. Outside of that, there's nothing to puzzle out and this is a straight-forward fight.
If you're still struggling, you need to power level some more. By now, you should be using Mt. Ordeals to push into the thousands.