It looks like they are gimmicking the fight here. Only the pet is on one side while tanking, so only the pet gets hit with meteor slash. I'm not even sure meteor slash hits pets, but if it did it's at a 90% reduced rate since it's AoE. The raid is only taking damage from the DoT and fel fire. All heals are focused on the pet. You don't really need to bring any tanks. A group could bring 8 dps and 2 heals and be perfectly fine. It's actually a really smart way to do this fight. It eliminates and simplifies a lot of the fight mechanics.