There is no real need to 3 heal this, 2 is plenty. The only damage to worry about is from Raining Blades which can (and should) be mitigated by a plethora of cooldowns. We also 1 tank it, but to do that you need to dispel Quickening immediately or the tank will get gibbed.
Didn't realise you could stack on the final phase. We just stood in a crescent shape at the back of the room with the boss in the middle then slowly moved him back. He died with plenty of space left.