Instructions
Engage in random battles to level up your party as much as possible prior to encountering Lady Yunalesca. If you're the type of player who avoids random fighting in an RPG, Lady Yunalesca will be very difficult. If you've been leveling up naturally throughout the course of the game, you shouldn't have much of a problem.
Stock up on spells and items like reflect and various types of armor prior to the fight. Lady Yunalesca will cast the zombie spell on as many of your characters as possible, so be prepared for that with a barrage of health spells and items.
Cast the reflect spell on as many of your characters as possible. This will make Lady Yunalesca's attacks bounce off of your characters and slowly drain her hit points.
Refrain from curing all of your characters once Yunalesca begins to cast the zombie spell. If you cure all of your characters of zombie, she will retaliate with an attack that will automatically kill your entire party. Cure two of your characters from the zombie spell, as Yunalesca will cast the curaga spell in an attempt to kill you. Since two of your characters will no longer be affected by zombie, curaga will heal them.
Follow a basic strategy of causing as much damage to Yunalesca as possible while simultaneously healing your party when the occasion arises. Eventually, Yunalesca will fall and victory will be yours.