How to Obtain One-Pump Chump
As many have been posting about this superpowered early-game Shotgun, I've decided to share a step-by-step guide on how to obtain the One-Punch shotgun, "One Pump Chump" all for yourself!
First and foremost, you need to progress through the Story to the point where you have left Pandora and obtained access to Sanctuary (III) and Promethea. On Promethea, progress through the "Hostile Takeover" mission until you reach the "Meridian Megaplex. Once you're in Meridian Megaplex, you should be able to enter Lectro City, where the "One Punch" boss is hidden.
From the entrance of Lectro City, hug around the left side, all the way past "Big Dock Energym," until you reach the south-east corner.
Make sure you hit the New-U station nearby.If you look around, you should see a stairwell heading underground:
Head down those stairs, and you'll end up in a run-down subway station.
Continue short ways down the subway station until you reach some stairs on the left, being sure to clear out the enemies that spawn.
Head up those stairs, and you should see a small nook.
Head in there and look to the left to find a room with a chain-lined fence.
When you enter the room, you should see five switches - Button, Handle, Switch, Wheel, Lever.
Using the five switches, activate all five of the TVs.
Here's the solution: Button, Wheel, Floor Lever, Handle, Handle If you've activated them correctly, a mini-boss called One Punch will spawn out of the doorway.
BE CAREFUL and make sure you keep your distance - true to his name, a single hit will put you into FFYL mode. Cluster grenades + fire weapons are pretty effective on him (in Normal mode). Remember that there's also a nearby New-U station right above the subway - so don't worry too much if you mess up.
If you're lucky, he'll drop the weapon right away, but I had to fight him 10+ times before he finally dropped it. If it doesn't drop from him the first time, just exit out to the main menu and you'll reload at the nearby New-U so you can spawn him again.