The Dimensional Arachnid is the fourth boss of the dungeon.  Keeping the adds from buffing the boss is important to this fight.

The strategy is the same for the Elite mode with the major difference being Dimensional Arachnid's health is 185k HP and damage dealt is stronger.

Black Bile – Avoid AoE
Research Assistant – DPS should focus down each time one spawns

Black Bile – Black ground target AoE which remains for a short time afterward, players who come in contact are damaged, hindered, and afflicted with Filth Undertow
Filth Undertow – DoT from Black Bile
Horrific Evolution – Stacking buff on the boss which causes each attack to do additional filth damage per stack, up to 215 stacks
Relentless – Buff causing Hinder and Impair abilities to last only 33% of their duration on this boss, and the boss will become immune to CC if too many are cast in rapid succession

Spawned adds:
Research Assistant (3.6k HP) – Spawn throughout the fight and buff the boss with Horrific Evolution

The tank should take the Dimensional Arachnid where it stands, and move it to a corner of the encounter area.

The boss will cast Black Bile, which places a black filth AoE on the ground which remains for a short time after.  Players who come into contact with the AoE will be damaged, hindered, and afflicted with the Filth Undertow DoT.  The tank will need to move the boss in order to try to keep out of the AoEs.

Throughout the fight, Research Assistants will spawn and buff the boss with Horrific Evolution, a stacking buff which causes each attack to do additional filth damage per attack.  As soon as an add spawns, all DPS should focus it down before returning to the boss.