Gas Spores will appear three times before Pungent Blight is cast. To meet the requirements for the achievment, all raid members must only recieve two stacks of Inoculated, not all three. Pungent Blight is survivable with only two stacks of Inoculated. Bringing back hp without anyone dying will be the hard part, as Gaseous Plague will return doing full damage right after Pungent Blight is cast.
Pungent Blight will do around 25000 damage with only 2 stacks of Inoculated.