Gain three stacks of Resin Residue by defeating Mantid Conscripts in Gate of the Setting Sun on Heroic Difficulty

Before the last boss Boss, go right instead of jumping down you'll see 3 big Mantid Conscripts fighting NPCs.

Kill those 3 Mantids and stay near them

They die. They explode and give all targets within the explosion a debuff called Resin Residue it has a 90 second duration.

Do one at a time.