It is an optional mission
You unlock it after completing Tier 2 Battle: Bar Room Blitz
This mission is timed you have 2:30 mins to complete
Gather at Rally Point
Kill badasses among the bar patrons
Turn in: Pyro Pete's Battle Board
Pick a fight in a Biker Bar. Tier 3 Difficulty
Mr Torgue: "Get ready to grab a beer and kick some read! It's time for a beatdown bar brawl! Just pick a fight with the drunks and stay alive."
Completed the Bar Room Blitz Battle
Kill everyone in Pyro Pete's Bar, in particular the Badass enemies.