Almost the same happened to me. But I got lost for a bit so I did exit the dungeon for a bit.Yeah he was really difficult. I used up almost all my curatives and I was lvl 50 party.
The entire time I was thinking, please don't let this be the boss, because I had spent like 2 hours in there killing lots of stuff and figuring out what to do. I shouted in excitement after I killed it. Seriously thought I'd rage quit for the day if I died there lol.