Somehow, after many, MANY hours of trying, I managed to get this done yesterday. As a paladin healer (specced arena 2s PvP) I found that I was the main reason we were not making this in 2mins.
Reasoning is as follows:
1) You do not "pop out" for assistance of insanity until you have taken down everyone bar one of your shadows is dead by your hand. I noticed this late in the piece.
2) I was taking far too long to kill the tank
3) I was not pumping out enough DPS to break out for assistance and still be able to make the 2 min timer.
What we ended up doing was that I respecced ret and wore most of my healing gear. This did a couple of things. It meant that my shadow did not cast healing spells, making it much faster for everyone else to kill their shadows. I also swapped my weapon/shield to a DPS weapon once inside insanity. I went for the ranged first, which dragged the melee with me and aoe'ed everyone down. I also left the tank for last (to get assistance on that one) - the tank was extra tanky as they were not going to get as many heals as usual. Everyone one who could cast a healing spell helped with healing when we popped out.
This technique got the damn achievement done as SOON as we did this. We had been failing on it for a good couple of hours, trying to figure out what was going wrong. As a holy pally, my shadow was too hard to kill and healed too well. As a holy pally, I didn't do enough DPS to kill my shadows fast enough.
Respeccing ret fixed all of these issues and we did 2 insanities and killed the guy with 10s to spare. I highly recommend any healing class respec to a DPS spec and wear a combo of your healing gear. There isnt a lot of healing required in this fight if you burn through it quickly. The slower you do this fight, the more you need healing. Trust your group to self manage somewhat.