The kill is with the buff, admittingly it's more a proper phase transit and P3 that makes or breaks this fight.
About choosing Fire:
Normally I play Arcane, but on some fights I play Fire.
The reason I'm playing fire on Putricide is simple, the fight is all about phase 3. Fire has Molten Fury and increased damage in the -35% region of the fight. It's also the spec that suffers the least from moving and since P3 is the phase where we move the most it's again +1 point for choosing fire.
Here's the fight but only phase 3 chosen - me and Avely is Fire.