You have to be careful this first fight against this boss could be pretty difficult. She abuses Hallation, which will reduce all your party members’ HP down to 1. The best way to defend yourself against Hallation is to have Frog use *Heal, or simply heal with Full Tonics. Don´t heal Frog (better use Full Tonics to heal, rather than *Heal) and use also Frog Squash when he gets down to 1 HP from Hallation.
If Frog will die, revive him and keep up the Frog Squashing. Have Crono use *Luminaire, and have Magus use *DarkMatter. Repeat this battle tactic, while healing when necessary until you win :)