i was in a flex carrying a friend with my 570 something hunter and we had an enhance 570 as well with us. The rest of the group was over 540 and we wiped 3 times. I left.
There's just something wrong with people. it's such an easy fight. i've only dps'd and holy priested it but it was simple imo in normal @like 538 ilvl. I really don't get it. it's no wonder blizzard makes things so easy.
In my experience, the stacking together usually leads to wipes because either:
A: People don't get out of the stack in time with their debuff
B: healers dispel it right away
But it's mostly A that I see pugs fail. You'll get the occasional pugs that die from breath, but not as often as the above.
If it's really your sticking point - in flex with 10-13 people (possibly 14...) the purple puddles which need tanking all spawn on the half of the room near the door where you enter. I know it sounds silly, and it is so counter-intuitive to anyone who raids normal/heroic, but my guildies and I do a run weekly with pugs on whatever alt needs gearing and since we figured this out the wipes are gone; mainly because we can cover them all.
The cone smash can still go in any direction, but you said you were marking those. Therefore keep everyone in one half, and hope that you have two ranged/healers awake enough.