<none of the above>, EU Guild on the Living Flame server, has achieved World First Molten Core Heat Level 3 in Season of Discovery. Their raid composition comprised mostly casters, with a heavy focus on Mages, Priests, and Warlocks.
Despite being one of the most familiar raids in World of Warcraft history, the difficulty spike from Heat Level 3 proved to be a significant obstacle to overcome, and there were many wipes for all competitors.
Fire Resistance proved to be very important, and even some of the weakest bosses like Lucifron proved to be quite the challenge.
The guild utilized a Weak Aura that tracked the cooldowns of their Duke's Domain trinkets, which provided the team with additional Fire Resistance zones.
World First kill clip from zeroji PoV.
This World First included the «secret» 11th boss that is only available on Heat Level 3, which before today, was a complete unknown.