High On Life

At the start of the fight, Use Gus primarily, whilst using Sweezy’s Time Bubble, as this will slow him down. Once you have taken a quarter of his health, the fight with Nipulon will become much harder. Nipulon will activate his shield, which will protect him from the bullets. At this stage, you won’t be able to land a successful hit on him.

Next, Nipulon will release a drug gas which will cause your character to experience hallucinations. You will be taken to a location in Blim City where you need to find 'Gene Zaroothian' and talk with him. Follow what 'kenny' tells you to do (walk into the black hole), then you will come out of the hallucination and back into combat with Nipulon. At this point, Nipulon will send out his decoys. These decoys are a different colour to Nipulon, shoot the yellow/orange Nipulon, (CHECK EDIT BELOW) as he is the real one. The hallucination will then repeat once you have taken roughly 2/3 of Nipulon's health, but this time, the story will tell you to find 'Lizzie', go back down the black hole once you have spoken to Lizzie and she disappears. You will then need to fight Nipulon again.

As the fight progresses, you will be placed back into the hallucination for the third and final time. This time, you will need to locate and pick up your guns again; then walk into the central black hole. A cutscene will occur after this, to which you will be teleported back to Nipulon's original location. You will need to push through a hallway of Nipulon's clones by shooting through them, once you get to the real version of Nipulon, you will need to stab Nipulon with 'Knifey' (DEFAULT KEYBIND IS RIGHT STICK DOWN ON CONTROLLER AND PRESS F IF ON KEYBOARD AND MOUSE). This will conclude this mission, therefore, the achievement will be given.

EDIT: Thanks @HoggyBear08 for letting me know, the Nipulon which you come across can be different for others, so make sure to pay close attention to Nipulon's style when you first see him.