At first Poisson seems like an easy fight, but when she reaches low health she’ll start casting a rain of hammers that is screen wide. These hammers can Freeze your characters and she’ll then throw a bunch of hammers to one shot your team.
There are a couple widely acknowledged ways to fight Poisson:
The first is to play defensively by switching to your healer once the raining hammers begins and focus on healing and reviving your party members, sticking to the outside of the arena where the hammers seem to hit the least. The reason you must control the healer is because an AI controlled healer will interrupt its spells to guard the falling hammers.
The final option is to use dolls to convert your other party members into Hubert to create a highly aggressive, heavy damaging, party that will chain Poisson to death with a bit of luck.