Use the ring on the 3rd infernal set. I don't care how long you have to delay it, it's worth it.
We do ring on 1st/3rd infernals and second dps potions+bloodlust on 4th infernals. If you don't die to those waves the rest of the phase is not that hard.
Video if it helps, we killed it about a month and a half ago.
https://www.youtube.com/watch?v=8pOTGezPLEE
Don't worry about the dps check to kill him before 5th infernals. Even with very poor dps in current gear if people aren't dead it's a very easy mark to meet. Worry about getting that far first.