We cleared 6/7 HC last week as a guild but decided to do Glory of the Firelands raider instead of the norm HC bosses because some people were on vacation.
We did The first 4 bosses on normal and then killed Bale and Domo on heroic. We went in this evening to see if Ragnaros would be there on HC but he wasn't there so we had to do normal instead.
So I was just wondering do all bosses need to be kill on Heroic for Ragnaros to spawn on HC?