He's very killable, just have your healer dps and do the interrupt early with tank cds popped.
Achievement and daily HM flashpoint work too.
Farmable boss is farmable.
Cleared all of them, they are fun and better/harder than WoW heroics but they're by no means unfinishable.
I don't really get how you're doing it. Our tank has like 20,5k hp, timed all CD perfectly and still took a 22k hit before anyone could even see a castbar. What are we doing wrong ? do we just need to know exactly when the cast is about to come and pre-empt it with an interrupt ?