Mutation occurs only if you get all 5 of the debuffs. I keep my healer decursing (that should clear 2 debuffs I think) on round robin and them just burst him down.

I have done it in the past just burning bloodlust (no decurse)and it worked fine as well. But it seems like the last 2 times the debuffs are spreading faster. Maybe just me I dont know.