This requires you to get shotgun kills. You may need to get them in rapid succession or possibly more than one kill at a time. The easiest way to do this is to play on Welcome to Hope, turn into the bathroom and flip the switch to start the fight. From the bathroom, climb through the vent to get to the security room. Inside there will be a shotgun. Now head back out into the bar and start shooting.
It may take a certain amount of kills or you may need to do it with speed. Either way, it may take a couple of attempts.
