I just beat this in my english playthrough and i have to say...it's been buffed up to hell(no pun intended) compared to JP version.
In JP version i killed this in 1hr 30mins with a PT of 3, average lvl 55. 2 DD + 1 Healer
In US version it took me 1hr 45mins with a PT of 4(Reddas as Guest), average lvl 67, 3x pure DD and 1 DD/healer
On to the details, to get to this you need to have cleared Vyraal Mob Hunt and got Dragon Scale. Go to Windmill #10 in Cerobi Steppe and give the guy the Dragon Scale to get Ageworn Key.
Now, go to Sochen Cave Palace and to the area with all the doors and stuff (kinda diamond-shaped-looking area on the map) and go to the west here to get to the locked door. Save accessory for Bubble Chain and get ready for a long fight.
This guy has around 6,000,000HP. Yes, 6mil. Each small bar underneath the big bar represent 100,000HP
Moves he uses:
Judgment: He will only use this once you take a certain amount of HP off him. 4000-5000+ damage and sometimes causes Stop, so get Chronos' Tear ready.
Invert: Switches current HP with MP. personally i like it when he uses this move cos i used up a lot of MP during my fight.
Stone Breath: Frontal AoE for 1000-2000 damage and sometimes causes Petrify
Rake: Single Target instant 1-2k damage
Note: His combo hits sometimes cause instant KO, thankfully it's like 1/25 chance
Spells:
Darkga: My Decoy tank had demon shield on, which absorbs darkness element, so it's pretty nice when he did it to me. but otherwise causes 2000-3000 damage
Water-/Fira-/Thunda-/Blizzaga: does about 1000-2000 damage
Keep someone on with decoy at all times and your healer with haste at all times because he does a lot of damage in a short amount of time. I had Reddas with Berserk+Brave+Haste on all the time cos he's a dick and doesn't use any healing stuff anyway. If your healer gets low on MP use Syphon on the Decoy tank for about 200-400 MP back, don't forget to gambit Charge.
His defense will go up once he reaches 50%, and again at 25%. Once he's critical, the fun really starts.
There are 3 stages when he's in critical, he moves on to the next stage after you take a few HP bars off him.
First Stage:
Hardest out of all the stages. He will keep using Rake, and only Rake. Gambit Decoy and Reverse on ONE CHARACTER ONLY and turn off your Cure+Esuna gambits. This is pretty much the only way to survive for this stage. Keep fighting till he starts using Stone Breath.
Second Stage:
Kind of easy. Turn your Reverse Gambit off and turn your cure gambits back on. This part might take a bit longer to do since i had all my characters with Esuna and they kept using it cos of the status. Make sure Breath only hits one person to mitigate this.
Final Stage:
Easy. He will use Invert then start casting a spell, so you have to be quick with your cures, otherwise he will kill them. Keep going and you will have beaten him ^^
I only wiped once and had to use my LV12/15/21 PT to revive my main PT. It was only because he used Breath on everyone then immediately followed with Judgment after, killing my healer and Reddas and Stop on my other characters.
Hope you'll have as much fun with this fight as I did =3