So You Can Kill This in a Way That Matters...

Mushrooms are untargetable untill boss is engaged, they are located on the inner edge of the path surrounding the arena.

Once picked up you can plant them with extra button and let the tank destroy it with Decaystrike.
Destroying a mushroom leaves a 20 sec DoT that stacks, so don't destroy all at once.

For anyone looking for the shrooms.
They are in the silence zone UPSTAIRS of the boss
You can run around and the area near the shroom has no silence zone.
Get to them before the fight, pull the boss, loot and jump down.
Plant them infight 1 at a time and let the boss hit them.
If they die you get a 20s debuff that hits kinda hard.

GL

Given the achievement name and its uses of mushrooms, this is a reference to a famous Tumblr @#$%post :

me holding a gun to a mushroom: tell me the name of god you fungal piece of !@#$
mushroom: can you feel your heart burning? can you feel the struggle within? the fear within me is beyond anything your soul can make. you cannot kill me in a way that matters
me cocking the gun, tears streaming down my face: I’M NOT !@#$ING SCARED OF YOU