Is it the Guardian Beast? Here's how to beat it
BOSS STRATEGY
Here's the recommended setup for this fight: Mythril Gloves acc (buy in Zanarkand shop), any type of accessory that grants elemental strike properties, Helios Guard GG - pass thru red gate to obtain Auto-Protect, Raging Giant GG - pass thru yellow and blue gates to obtain Berserktouch - then inflict berserk on Guardian Beast to never see Curse and Damocles Photon. But do note that it is quite resistant to berserk; if you have thief with mastered Soul Swipe, use it instead. It works like a charm.
One of greatest nightmares makes its return. Fortunately, it's way easier than it was in first release. It's very predictable and it has an attack pattern it strictly follows as well. Its swipe gets one for about 100 damage, and Damocles Photon hits all girls for 1/2 of each girl's MAX HP. Use Cura on all girls immediately! If it uses Curse, let it be. You must concentrate on wasting the boss before damage piles up too high for you to handle. It's also very recommendable to break this boss in various aspects, most significantly in power field to lessen damage it does even further.
Now that I've covered damage aspects, let me fill you in on boss' attack pattern. It will use three swipes in a row, then use Curse on one girl - rinse and repeat. If all three girls are cursed, boss no longer uses this attack. Furthermore, once you bring its HP to 962 HP or below, you'll start seeing Damocles Photon, its most dangerous attack. Attack pattern is implemented again - three swipes, followed by Damocles Photon, then repeat. Note that damage from Damocles Photon can be halved by shell status.
As you can see from info I've dug up, you're fairly safe until boss loses 2/3 of its HP. Up to that point you need to worry about keeping your HP only above 250 HP (just in case it manages to pull a critical on your girls). But as soon as it enters Damocles Photon phase, pull out all the healing you can and cast Shell on all girls. Get that white mage out now.