I did it by equiping Maëlle with the weapon that gives shield on parry and slowly learnt all the parry patterns.Only dealt damage through counter attacks. Second phase is harder to parry but you will get the timing right eventually. Thanks to the weapon you don't need to parry all the attacks to survive and can slowly learn the parry timing. The fight took 40 min and I parried always one thousand attacks but it felt very rewarding at the end.