I was talking to our sentinel last night, and we're curious: is there a reason you didn't just leave Stormcaller at 10% and nuke Firebrand? That way, you'd only need to heal through a soft enrage for a few seconds.
Yes, two reasons:
Firstly the Double Destruction actually seems to work (as it does in HM/NiM modes) after the last shield phase. On a few attempts we had Gosia stunned and shot up in the air after getting hit. Looking closely at the combat logs there seem to be two different forms of Double Destruction - "Double Destruction: Single Destruction"
which pretty much doesn't do anything and "Double Destruction"
which is lethal.
Secondly it is actually easier to heal through the soft enrage, being one consistent and predictable source of damage than it is to heal through the cleaves done by both bosses on the entire group at random times (sometimes simultaneously).