The first time I did this boss it probably took me about 8 minutes to down him. I ended up with just my druid healer and my paladin left up and slowly DPS'd him through his last 30% or so after losing most of my team.

I've found that popping fire elemental during the first insanity can help to get all the adds down. Also manually going through each toon and targeting your healer mirror image first can be a really helpfull.