Siren (Final Fantasy V)

Siren is a boss in Final Fantasy V. Tricking the party into losing their will to fight, Galuf breaks the spell thanks to his amnesia. As such, the Siren takes a more direct approach to taking care of the party.

Strategy
While Alive, Siren is weak to physical attacks, but elemental attacks prove to be ineffective. Have Knights attack her while your Black Mages defend. She will attack using Blizzard spells, while trying to put your party to sleep and silence them. She will also cast Haste on herself and try to cast Slow on your party. Most annoyingly, she'll heal herself occasionally.

While Undead, the opposite applies, so have Black Mages cast Fire spells on her while your Knights defend. If you have any White Mages, have them cast Cure spells on her too, since this will hurt her now. Siren will now physically attack your party for some heavy damage. She also has a Poison spell, so make sure those White Mages have Poisona.

One odd thing is that she also has the Libra spell. Don't question it, just attack her while she wastes a turn.