We use a similar setup, i specc for thunderstruck and gag order on this fight. No problem with survivability whatsoever, i use all four movement cooldowns to greatly reduce the time i spent outside of melee range. I pull the first 2 adds and wait for the third to spawn while aoe'ing all of them down passively. One of our ranged just snipe the lowest and continue to do so until 5-6ish stacks with the assistance of a second ranged if needed, we pop heroism and only afterwards it gets a little messy. Until that point your only competitors for threat are the healers, at that point you'll have to start killing the last two spawning adds ASAP to keep the stack rolling.