Return to the entrance of the castle again, and from here head up to the chandelier, which will generously give you another ride down to the lower part of the chandelier room. Head up and exit via the double doors at the top of the screen to reach the courtyard and search along the left end of the fountain to find an otherwise-invisible Treasure Vault Key. There's no finesse to this, just mash [TALK/CONFIRM] until you pick it up (see the included screenshot to give you an idea of where to look).
Return to the main hall again - use the party switch circle to get to your secondary party, have them exit, then re-enter the castle to do this quickly - then head up to where you fought Sphinxaur/Sphinxara and exit through some doors to the west. Yeah, new places. In the next screen, go down some stairs and pass through (or around) another green party switch circle and go through another set of doors to find yourself in a hallway. Go through a door to the left (see image, as this door is pretty hard to see), which will use up the Treasure Vault Key.
In the room beyond you'll find four boxes, some open, some closed. Bit of a puzzle here, but it's not too bad, let's label the boxes #1 to #4, going from left to right. Open box #2, close box #3, open box #1 and finally open box #4. When all the boxes are open, Catoblepas will appear.
Before you engage, junction Thundaga to your Elemental Defense or otherwise ensure you're immune to thunder damage. Junction Blind to your Status Attack and either Water or Quake to your Elemental Attack, then equip Auto-Protect if you have it, or otherwise just ensure you can cast Protect. These adjustments aren't essential, but they'll all serve to make this fight easier.
|Weaknesses:||Earth (200% damage), Water (200% damage)|
|Resistances:||Gravity (immune), Thunder (absorbs)|
|Status Resistances:||Immune to most status effects; Darkness 40% -- Slow 80% -- The End 0%|
*Only if you didn't draw Alexander from Edea during the War of the Gardens section of the guide.
Catoblepas is, compared to the other monsters you've fought, something of an oddity. First, its Vitality and Spirit stats are just bad, no matter its level. Seriously, these stats start out as low as 4 and 2, and top out at a mere 46 and 32, respectively. That being the case, you can dispense with Meltdown for a change and just rely on melee attacks - especially if you've junctioned Earth and Water to your Elemental Attack, which Catoblepas is weak to.
As for its offense, Catobplepa will use a variety of thunder attacks (Thundaga and Thunder Summon) to deal damage, which you can negate by junctioning Thundaga to your Elemental Defense. Thundaga is rather weak, but Thunder Summon is... well, basically Quezacotl sans Quezacotl, and even at low levels it can deal well over 1,000 damage to the entire party. No good reason why you shouldn't turn that damage shouldn't be healing, instead.
Bereft of that, Catoblepas only has physical attacks and its immensely powerful Deadly Horn, which is dangerous enough. If you have high HP and Protect you should be fine, but it's definitely something to be wary of. You can improve your odds by inflicting Darkness on Catoblepas, so either junction Blind to your Status Attack or cast the Blind spell on it until it sticks. If you've got Tornado junctioned to Eva-J, Catoblepas will probably miss with all its attacks regardless of whether it's been blinded or not, making Catoblepas incapable of doing much besides healing you.
When Catoblepas is defeated it'll unleash Meteor, which can deal significant - but likely not critical - damage to the entire party. Just keep your HP up and you'll be fine. Before defeating Catoblepas, draw Alexander from it, assuming you didn't obtain this GF earlier during your second fight with Edea. This is, naturally, your last chance to acquire this GF. You'll also score a Status Atk item, which will teach a GF the ST-Atk-J ability, if somehow you still need that.
Finally, you'll be able to unseal yet another power, and by now you should be down to Save and and GF powers. If you unlock GF you'll have all your battle powers back, but Save is arguably more handy if you're keen to fight Ultimecia, as it'll allow you to use the Save Point just before confronting her. Convenience, or battle options? Your call, but since we plan to go after Ultimecia's last minion either way, it probably doesn't matter too much.
Speaking of which, it's time to hunt down Tiamat.