I did them when the feature was still pretty new so I don't remember precisely how I did it. But I was BM with a spirit beast (ofc BM was much better than the other specs at the time) and I only DPSed the one that was outside the shields. Whenever he went inside I switched to the other one who was then outside. Also make sure that your pet has aggro and takes the dmg from that cast ability that they do. Use glyph of mending if you are having trouble keeping your pet alive.
Managed to kill Disruptron 5 minutes ago, I had 18k hp left at the end, before the fight I asked a priest for stamina, without that buff I would still be on it, I think this is my best achievement in wow to date, I dont know if anyone knows how to check the wipes on this because my DBM doesnt show it but I know it will be several hundred. Such a challenge!