Shemhazai is arguably the most formidable boss you’ve yet faced. Equip Rose Corsages on your healers, and don’t rely on weapons or spells; Shemhazai is resistant or outright immune to most elemental attacks, and absorbs Wind. Just try to stay alive and in the fight. As Shemhazai nears death, her Defense will climb rapidly and she’ll begin hitting characters with Disease status at every opportunity. When things reach this stage, blast her out of existence with Quickenings or Knots of Rust.
Annoyingly, Hydro can hit a character with Curse, inflicting Confuse, Sap, Disease, and Poison simultaneously. You’ll want as many Remedies as you can get your hands on, as well as the Esunaga spell. Hydro is undead, so it absorbs Dark spells.