Professor Putricide helps Festergut by sending Malleable Goo at the raid during the encounter.
Melee stay bunched up in 2 groups, each group on one side of the boss.
When you see small green puddle under your feet move to the other group.
(notice in the video how the melee switch left/right side of boss - it's tank pov but he moves with the melees in the beginning)
Get all 3 spores, even if it means taking some malleable goo.