This Esper has a very annoying habit of Silencing your party. Make sure everyone has an Anti-Silence Gambit before fighting. When the battle begins, immediately Dispel the Boss to remove her Haste. Start a chain of Quickenings to deal a good amount of damage quickly. After she uses her Mana Spring ability she is weak to an element, but absorbs all the rest. Only use spells on her if they are non-elementel. The Esper also casts Syphon, stealing some of your MP, but it is not too much to worry about. Keep attacking and healing until you defeat her.