hey i do the flamethrower too on my own wish in my group as elemental. i get around 35-40k depending on the other flamethrower.
this is my HC strat (not mythic !!!) just do dmg untilaround 50% boss hp maybe or even more. so that you have enough green shit to clear.
then u have to try to get 3 patches with turning on and off right away. just 1 global of flamethrower !! get 3 patches if possible. when you
are about to overheat go back to damage boss and go stack again when your thrower calmed down. i get around 25-30.
unfortunatly my raid doenst let me stack more before killing boss but yeah , with my mage alt its 10000 times easier xD