Porom will be assisted by the Elder of Mysidia. He has access to all White Magic but Holy. Like Palom's fight, this is mostly scripted. Porom should equip a bow and the Elder cast Haste and Berserk on her to increase her damage output and speed up the fight, which ends with the Mysterious Girl summoning Ramuh to defeat them.
Golbez and Fusoya will fight the Mysterious Girl, who has summoned Asura at the start of the battle. Asura cannot be defeated and their attacks must be focused on the Mysterious Girl. She will attack them with strong magic and Binding Glare, which inflicts Paralyze.
Fusoya should cast Haste on himself and Golbez and Slow on the Mysterious Girl, and place them both in the back row to reduce damage from Asura's physical blows. Asura's presence means the Mysterious Girl is treated as being in the back row, reducing damage from physical attacks, so magic is most effective. Use level three magic of any element from both Fusoya and Golbez, healing when needed.
The fight is nearly the same as the last, but the Mysterious Girl has now instead summoned Leviathan. Leviathan will use Tidal Wave while the Mysterious Girl uses the strong spells she had from the previous fight. Tidal Wave is strong, and it can KO a party member if they have already took a lot of damage. Eventually the Mysterious Girl will cast Osmose on Leviathan two turns in a row, signalling she is preparing to cast Meteor.
The same strategy for the first battle fought in the crater still works, but Fusoya should focus more on healing both him and Golbez, and the fight will go quicker without Asura buffing the Mysterious Girl. Haste and Slow help and Shell is also helpful for reducing damage, if Fusoya has a free turn from healing to cast it.
The party the player is given consists of Golbez, Ceodore, Rosa, Rydia, and Edge. The Mysterious Girl retains her powerful magic from previous fights, and can summon Bahamut to use Mega Flare, dealing heavy damage. Though Bahamut will not remain, she can summon him several times if the fight is long enough, and she has substantially higher HP than in previous fights.
Ceodore should use Awaken at the start of the fight and use it to its full potential before the effects run out. Rydia and Edge's best strategy is to use the Band ability Inferno, which will do around 1,500 - 2,000 damage to the Mysterious Girl. Golbez's best magic to use against her is Flare, if he has learned it. Because Rosa is the only healer that joins the fight, she should use healing spells every turn. If she has learned Curaja from Kain's Tale, she should cast it on everybody.
Another method is to use Rydia and Rosa's Holy Burst Band, with Edge using Healing Pill and Ceodore healing when needed. Ceodore and Rosa can also Band together to use Divine Heal on the party, granting them Shell status to further protect them if the fight occurs during a Full Moon.
When the Mysterious Girl nears death, she will attempt to raise her power so she can cast Meteor. This is the party's chance to kill her quickly before she attacks, which should not take a lot of damage to do. If the party isn't quick enough in killing her, she may summon Asura, Leviathan or Bahamut to aid her.
Bahamut will be summoned at the start of the fight, and this time the player must target him. Rydia must be in the party or he will begin the battle with Mega Flare, killing the party instantly.
The Mysterious Girl uses her magic as in previous battles, and has gained the more powerful Bio, Quake and Tornado spells. Bahamut will count down to Mega Flare each turn. As the countdown ends the Mysterious Girl will cast Black Hole to remove the party's buffs, including Shell and Reflect. Thus, the player must simply have their party at a high enough level to survive Mega Flare directly.
In the 3D version, one should keep Rosa, Ceodore, or Porom alive. (Rosa, or Porom are suggested because at that point they should have learned all White Magic spells). The party shouldn't attack when Bahamut starts counting because the attack bar needs to be filled with white to have enough time to cast Reflect. Right after the Maenad casts Black Hole the party can cast reflect, reflecting Mega Flare. Having Reflect on before Black Hole can reflect the Maenad's spells.
If the player defeated Asura and Leviathan earlier in the Subterrane and reacquired them for Rydia, they will appear during the fight to call out to Bahamut, and he will return to his senses and attack the Mysterious Girl. After the fight, Rydia will regain him as a summon. If the player did not acquire both Asura and Leviathan, Bahamut must be killed and is lost and the Mysterious Girl will be defeated with him.