Sonya Mainyu/Labyrinth of Chaos

Sonya Mainyu is the yoma/apoptosis counterpart to Sonya's alternate forms.

Biography
She is a super boss encountered in the Labyrinth of Chaos. Like Sonya Mazda, we know nothing about her. You cannot talk to her, there is no rape scene if you're beaten by her, and you can't beat her (well, you can beat her, through exploiting a skill called Vicarious Clara. It provides a shield which blocks all damage to the user for the next three attacks, and can also be stacked).

Strategy
She is one of the harder super bosses in the Labyrinth of Chaos. Fight her like you would any other super boss. Stack Vicarious Clara, stack your stats, debuff hers. Then attack her. However, you want to keep these things in mind:
 * She is completely immune to physical attacks. But, she is weak to plenty of the elements in exchange. Magical attacks are preferable to anything else, as they will deal more damage, while also being guaranteed to hit. That helps deal with her evasion. Keep in mind that any certain hit attacks which use physical damage (such as Makina skills), will heal her instead. Also, she is immune to mana drain, so that won't be of much use here, obviously.
 * While magical attacks are preferable, it is sometimes necessary to use something else. Using other attacks sometimes is required because Sonya Mainyu has a self-buff which grants damage reflect, like Carbuncle, this effectively turns your attacks against you. In this situation, you want to use non-magical attacks, but keep in mind that physical, bio and sonic attacks will still heal her.
 * She can inflict both party wide instant death, and Ascension. Bringing a Fox Geisha, like Nanabi for her Anti-Death Song is vital to countering this. If you don't, expect a short fight.
 * She also has certain hit attacks which inflict burn, shock and freeze. Vicarious Clara can deal with these attacks, as it will prevent them from landing in the first place. But if you would rather not take the risk, Genji Shield provides immunity to all of these status effects
 * She has an attack which inflicts single target blind. Again, Vicarious Clara should be able to protect you from this, but if you'd rather not take the risk, Genji Armour provides immunity to it.
 * Sleep can be helpful here. Sonya Mainyu isn't immune to it, and it will prevent her from taking any action for up to six turns if it lands. However, attacking will wake her up. So don't wake her up. Instead, use that time to buff your allies, heal and summon Vicarious Clara. But you'll still need to debuff her defence, so waking her up at some point is necessary.

The video will provide a tutorial on what you should do exactly, and who you should bring to the fight. The guide will give you the necessary information on gear, races, jobs, etc.

I got most of the information from the guide and the video. Both are extremely helpful, and I'd highly recommend you check both of the guides if you're serious about beating any of the LoC bosses.

Trivia

 * Nothing is confirmed about her in part 1. Only that she is possibly Sonya, but she could be Karen for the all the information we know. I say, Karen, because Adramelech is likely Karen, and both Adramelech, Sonya Mainyu, and Sonya Mazda possess a horrific scar on the side of their face. But whether this is a trait shared by all of Sonya's families alternate forms can't be said either.