The Pain Drain is located in the northern part of Torn Plains. In its surroundings you will find Delta Charging Station and an Abandoned Garage.
To get to this location you need to enter the pipe, located on the hillside. This place is shown in the screenshot above.
Remember that your goal is to kill Sackhead. However, on your way there will be hordes of mutants. Kill them so they won't disturb you during the main fight.
Move along the channels to get to the monster. This the road is simple and straight. All other paths are blocked.
When you see a big hole in the floor, jump in. Along the way, kill all the enemies that rage at you.
Your opponent will come out of a huge pipe and jump out in your direction.
Sackhead will attack you if you get in his way. It is best to shoot him in the back and avoid his attacks. He has a small amount of health points, so you will kill him very quickly.