I'm all for hard, but this wasn't healable like this. Even if I had unlimited heat I would not have been able to heal through it, his DPS was about double my HPS spamming my strongest heal nonstop. If he hit the same person twice they went from full health to dead in about 2 seconds.
I remember him sniping in beta, but he would do it occasionally mixing it up with the flamethrower and the knockback. Now hes spamming it non stop after a few seconds into the fight and never using his regular attack, flamethrower, or anything else.
I found it very weird because his damage output was about three times as much as any boss we'd fought previously in any flashpoint. It felt very unusual and in beta we never had any problems like this, he hurt when he had his adds out but not to the point where you couldn't even heal it with cooldowns up.
Its not "unhealable" as I've done it as a Merc at appropriate level.
1. MOVE OUT OF THE FIRE. Seriously. If the flamethrower is on you, MOVE.
2. Turn and kill adds ASAP. If you have 2, CC one of the smaller droids that come out of the tubes
3. In "Massacre / Slaughter" mode or whatever HK calls it, he will focus one person. Heal that one person.
Make sure you keep gas on the ground and Kolto Missiles flying for the extra heals.