Oh that mean Piston. Our show of friendship was predictably thrown back in our face, and now we're all alone in a dank and murky room. No worries, it's not the gulag, and the exit is just a few jumps away...
Simply look to the east (after a little ammo pilfering) and follow the stairs around the room anti clockwise, dealing with any enemies you meet along the way.
Once outside listen to the blabbering and head forward towards the 'Scooters sign', grab a car and then drive north. We're heading to The Beatdown.
Once in The Beatdown it's time for some good old fashioned street fighting. Lure enemies to you and dispatch in the same way you would any band of psychos, aiming for the southern Checkpoint.
Again, try and hold back to lure melee enemies into range before dealing with ranged foes via sniper rifle justice. Eventually, after cutting a swathe through the waves and waves of foes, you'll come to Pyro Pete's bar. Hand the quest in at the nearby battle board to finish this straightforward mission.