Sphinx/Paradox

Sphinx is a legendary Yoma, who was alive throughout the Great Monster Wars and is still alive to this day, making her one of the most ancient beings in the Paradox universe and the original timeline. She is one of the super bosses and resides within the Pyramid. She is also in the Labyrinth of Chaos, albeit in an even stronger form.

Biography
After Luka and his companions meet with Salaan to discuss how to restore Sara’s sanity, she sends them off to find Sphinx, who could provide that information.

When Luka meets with Sphinx, she tells them that she has no interest in the affairs of humanity, but when they explain to her that Sphinx is the reason why this is happening, she perks up. After Sphinx explains to them that her humanity cannot be restored, Luka is distraught. But she does tell them that her sanity can be returned; by accelerating the process. Luka and Sonya are naturally confused at first, but after Sphinx describes to them that her loss of sanity is tied to her becoming a succubus so quickly (due to the Lilith Sisters), they understand.

To do this, Sphinx gives Luka a phial of her blood, tells him to have Sara drink it, and she will go back to her usual self. After working out how to get Sara to drink the blood, Sphinx tells to go away as she “likes her peace and quiet”.

After this, Luka can challenge her and can recruit her after the quest for the orbs, as she decides to support him deal with his great burden while also wanting to move herself.

Monsterpedia Entry
''“A mixed beast monster that guards the Pyramid, believed to be older than 1,000 years old. Through her long life, she has come to wield powerful magic. Through various circumstances, she has become the judge over the [Dragon Seal Trial]; however, it seems she despairs at her role and lives her life in immersed hopelessness.''

''Those who fail the trial are violently raped, then eaten. She forces her prey to be entranced by pleasure as she swallows them whole and slowly digests them inside her gigantic stomach.''

Her enormous magic, built up over hundreds of years, renders her all but immune to any type of sealing magic.”

Attacks

 * Attack - Normal Attack, One Enemy
 * Beast Rampage - Physical Attack, Random Enemy, Lightning Attribute, Shock 10%, Attack Four Times
 * Arc Obelisk - Magical Attack, Random Enemy, Fire Attribute, Burn 10%, Attack Nine Times
 * Kiss of Death - Physical Debuff, One Enemy, Death 75%
 * Hand of the Earth Mother (M) - Pleasure Attack, Target: Luka
 * Hand of the Earth Mother (F) - Pleasure Attack, Target: Female
 * Mouth Offering - Pleasure Attack, Target: Luka
 * Isis’ Lovemaking - Pleasure Attack, Target: Luka, Trance 75%
 * Ancient Law of Ecstasy - Pleasure Attack with Bind, Target: Luka

Strategy
First thing, you're roughly supposed to fight her when gathering the Six Orbs. You can fight her earlier, but her rewards aren't more interesting, and she'll only join you then.

As a whole, she's not horribly tough. She possesses decent hit points, good status resistances, and no glaring weaknesses. Physical and nature elements are a good way to go, and, surprisingly, dark is more effective instead of holy. A taoist with another nature-element based character and a few buffs can be enough to finish this fight before being in any real danger.

She possesses solid pleasure attacks that can do a decent amount of damage to Luka, but when she does that, it means she isn't using one of her two more damaging skills, so enjoy the opportunity to heal and buildup Luka's endurance if it isn't already done by that point. Beast Rampage and Arc Obelisk can, with a little bad luck, take down two or even three party members in a single turn. If you have a wing harpy with sky dance, you will easily be able to reliably mitigate her damage. If you have magic reflect like Summoner's carbuncle, she will even help you take down herself.

Evaluation
"As expected, you came here. Well, I knew this would happen. Sphinx is an opponent you can take on as long as you have a suitable level. She uses powerful Pleasure, Fire and Instant-death skills... Fire element isn't very effective and status ailments will almost never work. There is however a small chance for Slow and Stop to trigger. Anyway, what you really need is enough power to survive her raging attacks. Training long enough is necessary in order to be victorious. As for what you should do... Avoid battle against an opponent you cannot beat. Now go, oh brave Luka. One day, you will be able to thoroughly destroy that abominable monster."

Trivia

 * In the original trilogy, Luka entered the pyramid as he was searching for Sara who wanted to pass the Dragon Seal trial, and he only fought Sphinx by giving a wrong answer to her enigmas. In both case, Sara is the reason Luka goes to the pyramid.