This is available after getting Etna.
After this achievement, there will be a new bill in the Senate, Another Netherworld's Overlord Appears!
. Pass that bill and the post-game chapter will open.
This is a straight-forward stage, with almost all monsters fusing or magichanging after the first turn. However, if you've been rushing the post-game characters, Laharl can absolutely one-shot Desco. If this is the case, you'll want to bait him closer to the portal before fusing another monster into Desco, using a Fighter to Fight Shot and double her ATK, and surrounding her with allies, preferably Beast Masters, before trying to one-shot with Yog Sothoth. With more characters at her level, the chances of rushing him and surviving are much better.
If you're still struggling, you need to power level some more, if not power level some different characters. By now, you should be using Mt. Ordeals to push into the thousands.