Sorry for the wait. It's been a while since I fought it though, so there might be things I got wrong:
- Your entire party is level 99, right? With chemicals, it's not mandatory, but it helps tremendously to be at least level 90+, especially if your party isn't custom-tailored to deal with it.
- Make sure to use its pupa phase to boost yourself as much as possible (like setting up Ignition for your Nightseeker, putting down dances and guards for your Fortress, setting up circles, links for your Land). It has 10000 HP, so make sure you time it right to benefit as much as possible from your buffs.
- Kill its limbs as quickly as possible. The Eye deals more damage the more of them are alive.
- Party Shield from your Fortress should help for survivality against the limbs.
- Snare Circle + Squall Volley deals good damage as well.
- Hygieias Bowl and Geo Impact are pretty useful Burst skills. Geo Impact in particular to stun it the turn it opens its eye so that you don't lose your buffs.
- Buy some elemental Mists to help diminish the damage from its spells.
- Eat food that buff your elemental resistance. Silver Carp or Dragon Carp in particular.
- Venom Throw should stick pretty reliability to it. Make sure to use it after it opened its eye. With Foul Mastery, that also means your Nightseeker will do more damage against it.
- Have your sturdiest member carry an anti-stone accessory if needed.