The Acid Gat Kit can be constructed on any of the three workbenches scattered around the map, provided the Zombie Shield has not been constructed in its place, or Brutus has not locked it down. The three components are a motor, a case, and an acid bottle. The motor can be found next to the cell containing the plane part, next to the Mystery Box location, or to the left wall next to Speed Cola (all in Warden's Office). The case can be found directly outside the Library (next to the cell with the hidden blue skull), opposite the Afterlife-activated door in the cell block, or under the staircase which has the locked Power-Up. The acid bottle can be found on a metal table opposite the Afterlife portal in the infirmary halls, or in the general area where the blood bathtubs are at.
The achievement is unlocked once the player has taken a Blundergat (or its upgraded version: the Sweeper) to the workbench where the Acid Gat Kit is assembled and converted it into the Acid Gat (its upgraded version being the Vitrolic Withering).