I just read up on quick demise tactics on wowhead and it seems it's possible to burn him down. If your gear is good enough. I'll def. be trying this tomorrow.

"Unless you push the boss below 33% during the first Insanity cast which is like 5 seconds, if you manage to do this (not that hard with heroism and good gear), then the boss will start casting the second Insanity and during this second cast, you will be able to target him.
Heroism should be still up, just burn him down as you did during the 1st cast"

"Chiming in with above post; just did the achievement and it was nothing but raw DPS.
Setup was; Prot warrior, Holy Priest, BM Hunter, Elemental Shaman and Fury Warrior, all Naxx25 geared.

We popped bloodlust and cooldowns at ~75% and he was down to 40% before he started casting first Insanity. He hit ~20% before he was done casting it and he instantly started casting second Insanity. We could still see him even while phased because he went straight for second Insanity, so we just ignored the clones. He died during second Insanity cast of course."