As Jacinda's health is lowered, she will begin making clones of herself. She will begin by saying "Sometimes there is one" followed by "Sometimes there are many". After that there will be 6 indistinguishable Jacinda's around the arena. You will need to find the "correct" Jacinda in order to continue the fight. Unfortunately, attacking her clones will restore her health considerably. So you need to be careful while fighting her.
To help you find the "correct" Jacinda, take note of how much damage your regular attacks deal to Jacinda early on in the fight. Her clones will take more damage than normal (about twice as much damage with a regular attack). What seems to work is doing a regular ordinary attack, and then it is easy to tell which ones are clones and which is the real one. Have party members call out when they have found the right one so everyone can attack it. Towards the end of the fight, Jacinda with move around a lot, so getting lucky with finding her and having a few party members (1-3 will do) knocking off the few remaining percent health on her seems to work, if you all can't find her immediately.
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