Operation Abyss: New Tokyo Legacy

Sometime after earning Double Wizars, there should be a side mission called "Ongoing Issues". If this mission doesn't show up, make sure to complete any other missions (including side missions) first. You have to head to Babyl Sphere C24 but before you do so, you have to defeat Soul Eater in Morimoto High-rise ?F and Soul Rebirth in Hinowa Abyss Sector T. See Whoops! and Might As Well Jump... for more information about these. Once these are defeated, the doors will open in Babyl Sphere C24. You need a Babyl Key Beta to open a door to re-fight Soul Eater and Soul Rebirth in battle.

Before you start the battle, you will be asked some questions. The answers are "Mu'La" and "Fight". Once you finish this fight, you can now fight Mu'La. You need a Babyl Key Gamma to open the door to fight Mu'La.

You should be at level 40 before fighting Mu'La, and have good equipment. I had my Knight use Aegis to protect the party, my Warrior use Concentration and then Gelicide for 7 attacks at once, my Samurai use Quick Slash III, my Physician either heal or use Light Sphere, my Academic use Brain Spire, and my Wizard use Concentration on the first turn and use Magic Burst with Atomwork (with many enemies) or Genesiswork (with only Mu'La remaining). I used Magic shell III, Burst air raid III and Fast Code III as the Unity Skills. After you win and you return to headquarters, the level cap will be lifted to 99, though at this point it really isn't necessary for any more trophies. You can refight Mu'La but everytime you do, Mu'La is more powerful, and you also have to use another Babyl Sphere key because you can't use Call Portal to this location.