WHM would be Cleric Stance+Stone 2 spam. Depending on the length of the conflags dotting them up might be slightly better, but if 40 WHM potency is the key between you killing fast conflags or not, your problem is either stacking too many folks for fireballs or your dps being undergeared/not skilled enough for the fight anyway.

I suppose the ultimate rotation for a 15 second conflag would be precast Cleric's + precast Holy, Aero 2, Stone II x 4, and Aero+Fluid Aura on the last GCD before it explodes.. Assuming perfect timing on the Holy, that's 1330-1355 potency (240 from Holy, 210 from full duration Aero 2, 170 x 4 = 680 from Stone II, 50 from the initial hit of Aero, 150 from Fluid Aura, and 25 more if the Aero gets a tick off before it explodes). It also burns a TON of mana and means the tank had better be IN the conflag, since you aren't healing him.