When your party decides to head to Sealand. You will come to the Iris Plain. Head to the west part of the Iris Plain and you will now see an entrance to the Underwater Garden and a save point. Go through the place and beat the boss to obtain Paracelsus' Round Table. After the war, give the Paracelsus' Round Table to Welch in the Craftsman Guild at Peterny and you will be able to play VS Mode. At first, you only have 6 characters and their 1P and 2P. To unlock others characters and outfits, you need to collect Trapping Cards.
To get Adlai, after Amina and Dion meet again, head to the Audience Room in Sealand and head out. An event will take place, you will be able to let him join after the event. As for Mirage, she will join you automatically at the Diplo.
*These two characters are playable in Director's Cut only*
After you beat Iseria Queen at Floor 211 of Sphere 211, you will obtain a Key Item. Now head to Crosel's room and use the Key Item at where Crosel rests. You will end up fighting Frei/Freya.
After you select the stage for versus mode, hold down L1+L2+R1+R2. You will then play the battle as normal, but with a random boss music. To unlock more boss musics, simple defeat a boss with that song.
To pick 1P, press 'Circle'. To pick 2P, press 'Triangle'. To pick 3P, press 'L1'. To pick 4P, press 'R1'. To pick 5P, press 'L2'. (Fate and Sophia ONLY) To pick 6P, press 'R2'. (Fate and Sophia ONLY)