The Slurry Amalgam

Encounter

• On any cast/plan shoots off an acid blast at the caster
• On any cast/plan emotes "A mass of slurry flies off the slurry amalgam."
• Every combat round emotes "The slurry in the room slowly flows into the slurry amalgam, increasing its slurry mass"
• Periodically does a form wide attack emote:

([92%]Vivi)<4807hp 2764sp 3240st> 

The slurry in the room slowly flows into the slurry amalgam, increasing its slurry mass.
the slurry amalgam's slurry rapidly expands and explodes, coating everyone in the room!
The slurry amalgam opens its mouth and lets out a piercing cry!
You are blasted on the head by the slurry amalgam's sonic wave causing middling damage.
You dodge out of the way of the slurry amalgam's first attack.
You are burned on the head by the slurry amalgam's acid causing heavy damage.
You are burned on the leg by the slurry amalgam's acid causing moderate damage.
The slurry amalgam is cut on the central-mass with your slash causing trivial damage.
You fail to hit the slurry amalgam with your second attack.
The slurry amalgam is cut on the central-mass with your slash causing paltry damage.
You fail to hit the slurry amalgam with your fourth attack.
Nister's Redemption from the Darkness glows brightly as it strips away the slurry amalgam's magic.
The slurry amalgam is vomiting blood.

([8%]Nister)<652hp 2788sp 3285st> 

• Can scream paralyze
• Doesn't regen very quick out of combat
• Aside from reacting to spell casts with acid blast, doesn't cast otherwise
• Headless so slit throat immune
• Bash and pierce immune

From the emote descriptions it can be inferred the strategy should be to be casting every round in order to delay the form wide attack as long as possible. Alternatively you don't cast at all but flee out whenever your hp reaches a certain threshold (e.g. 4.2k hp for 130AR~ tank). Don't recommend this if tank does not have a spiderbite bracelet to prevent paralyzation.