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.
Pandemonium is fond of inflicting Blind and Petrify on you, which necessitates an appropriate response. Beyond that, a character with a good shield in hand has little to fear from him. When he reaches 50% health, he’ll use Perfect Defense to briefly immunize himself against all damage. This is a good time to renew your buff spells or restore your MP.
Use Bubble, Protect, Haste, and Regen to stay in the game against Slyt, who is fast, powerful, and generally very dangerous. The easiest way to beat him is to throw Oil spells at him until one takes hold, then cast Decoy on your most durable character and tag in a spellcaster. A few doses of Firaga against an Oiled Slyt should destroy it in no time.