Yep I got everything.
You're hp is draining like crazy during this fight an he's immune to most statuses. Just buff up on everything to start and wear Blackbelts to counter disable. Use whatever heavy physical attackers you have and pound his ass. Casting arise instead of playing catch up with healing spells may help too. Just set up your gambits properly - also using items can be much faster so you may want to use a few high end healing items from time to time.
Get him down to half or more and then nail him with a good quickening chain. His hp isn't that great. Just keep on him as much as possible, maybe have one character hang back a bit with a bow and handle as a healer (maybe even take control of that character for the fight and let the other 2 wail on him with the best weapons you have) regen on armor is helpful too!
Your party should be able to take him at this point.