"The prodigious waste from the Serpent forges has to go somewhere, and it all collects here: a toxic trough eaten into the hillside, where slag bubbles up from the ground and scorches our boots. Delightful!"
Upgrade all pyroclast for the Volcano Towers to level one when you can, then upgrade the bulls eye for the Sniper Towers to level one. After you have all that done, upgrade pyroclast to max and the two outer Sniper Towers to max. The inner Sniper Tower should only be upgraded for headshots.
Related Quest Edit
|Easy||Dark Sinkhole||Win "The Sinkhole" with 3 Stars without using Illuminator Towers|