Hi! I play on EU and just returned to the game again. I played it 10 years ago until HoT (took a break and moved to FF14) and again when PoF came out. So far I am enjoying the game again but found I have no one to play with and couldn't find like minded people. I work a lot and play other single player games when I can. so your guild sounds like a good place for me to be in and manage that balance haha. hope I can join!
For people looking up this issue still, I had to use my Specter to Condi him a lot and use the Volatile Ley Energy to start attacking him and little by little lowering his HP at around 10% the core appeared again and I killed him.