The fight (on mythic btw. Not much point doing it on HC) is a bit like Cenarius in how much you can shorten it by removing add spawns. Fastest kills take about 4 minutes. The boss hits 50% about 1:30 in, before anyone comes back up spawning adds. Adding a little bit of gear and a comp that doesn't worry about the long term, you can probably zerg it with just one tank. The only limiting factor for that is when that tank hits 100 torment, and a BrM dodging the majority of the hits can facilitate that.
Also: One tanking is going to be more fun than this stupid down phase.