What the fuck? I don't know what your level was like versus mine when I went in there, but that was far and away the hardest boss of the game for me, even in a full load of fire-resist equipment. Though I don't think I had whatever armor you guys are talking about. It was close to one-shot kill on me when he did the big fire blast thing, and once he's awake, you have to be at your quickest to get from the entrance to that area toward the back where you can dodge it. Looking at videos, I suspect this may have been patched or altered, because it was a MUCH closer call for me every time I ran in there, and he was never standing as far back along the ridge as he seems to be for everyone else.
Damage isn't the hard part with that guy, surviving long enough to do the fucking damage is the hard part. Or was for me. Actually, I think I did better when I first woke him up than I did on the next 5 or 6 runs. At that point it was all just figuring out how the fuck to survive the initial blast and what patterns he runs after that. I suspect you may have gotten really lucky and avoided the worst and killed him quick enough that he just didn't do anything too terrible.
But yeah, looking at YouTube vids, almost every single "how to kill CD" one has people killing him when he first wakes up. If he's already awake, it appears you can get him to lunge at you and launch an arm behind the shield rock, which he didn't do with me. He'd just stand on the other side and keep blasting (which is tricky as it can even hit you like at the edge of the rock where you think you'd still be safe), so I had to run OUT and be exposed to do any damage to him at all, which always risked getting slammed on the head with his arms or that he'd do the big fire blast and one-shot me or leave me nearly dead.
I wonder if this has changed or if it's just a positional thing where he was standing in a certain spot or something. Either way, this dude kicked my ass a good dozen or so times.