Actually, when I got him last time I was playing around with my DPS macros and had taken purge out, and I don't have purge keybound to anything by itself (lame, I know). I just jumped when he dark shrouded to stop attacking him - it only lasts a couple of seconds- then continued when the buff faded. So, if you have enough group dps and background healing, you can still zerg him without messing the adds. For reference, I'm doing about 14K group DPS on boss fights currently, and getting around 400 HPS per toon from totems. Think I usually drop one chain heal near the end when the adds stack up as well with this.