Final fight in main story, after you beat her you can go back to the place you beat her, get her soul, and place it in the soulless body in the abandoned lab in the Barrelzen Slums, at that point she will join you as bed companion and be available for Arena matches (shes the highest levelled Arena opponent as the final boss).
Its a pain in the ass to fight her so many times since shes really tanky, but I strongly recommend to do it anyway if you wanna collect all the good cards and weapons, they stack too, so its best to farm at least 3.
In Arena you unfortunately cant use Mema for the drop rate boost, so its good to get 50/75 Luck (not sure which of these triggers it, you get a notification though) which will give you an additional +100% item drop boost.
If you're having trouble in combat, 150 INT 120 VIT 75 LUK is the way to go for easy quick fights, get and max: Tier 1 SP and HP recovery passives, Heal, Magic Amplification, Mind Break, and Nightmare Breaker.
You should still have enough skill points left for maxing all strip and steal skills and a couple other skills, Grand Sexulus is a good option if you dont just want to spam Nightmare Breaker, it can hit over 50k on Versebub, and Versebub drops a card that negates Brum Bust self stun, so you can switch Grand Sexulus out with Brum Bust when you have the card if you want to, and you wont have to get Warp Shot too (using 4 skills every single turn is too annoying, only reason I dropped Brum Bust after unlocking Tier 3 jobs).