I guess its an easter egg kinda thing because there is no questmarker for it. When you visit the slums, where you can do the healslut, ratshreddres, etc. bounty there is actually a ringleader type of guy whom you can talk with. It doesn't have a questmarker. You talk with him, he tells you to fight all his men. That is pick a fight with each type of enemy there is. It all triggers a fight. You defeat all waves in one go and you can pick a fight with this ringleader dude (he is the one at the end of the road) and if you beat him he drops the gun.