Maybe I got it wrong and it was something like 9k > slightly below 8k with a break, but either way, I'm pretty positive he healed for that much at the beginning. Maybe you have Wakka's Pentabreak?When I just fought him, his curaja was 7960, nearly the same as d250 without a mind breakdown.
I'm not sure how exactly works to be different for us, but I was using Wakka.
edit: so my first try at d300 got him to 40% but I ran out of soul breaks and abilities. I need to change my strategy all around.
I may use wind for a medal but ultimately depend on radiant shield and a bunch of healers.