Yeah, I think you are right Slobby.
My revised plan (after talking it over with some other guys) is as follows:
* Do phoenix power first (when he has no fat, in the hope it won't inspire him to burn gems)
* Do power of the spheres second, and make sure he has a spare astral gem to burn
* Then lay down the fire storm
Of course, my dude is actually dead due to the failure of my last attempt, but in my next game this is what I will do