after 10 tries finally the mushroom died.
this is what i did :
i stacked 18 constitution on my pc (rauatai biscuits, resting bonus, etc), inhaled spores and severed the bonds between the mushroom and the vithraks.
The main problem however are the tentacles, but if you kill the radiant spore the fight is over. So rush for the big guy, petrify him with aloth and dps him hard, ignoring all the tentacles. the someone will fall, but you should kill him quite easily.
potions, scrolls and summons helps a lot. (alacrity, scroll of valor, scroll of defense, shadows)