Confront Artie Higgins

After inflicting enough damage or completing all objectives, talk to Purdue to get the location of the racket boss. Make your preparations then head there. It will be easier this time to enter from the bakery side of this hideout. Like before, it will be better to perform stealth kills as much as you can. If ever you get detected, the best place to ambush your enemies is in the doorway inside the bakery. This will make your enemies go through a single route, which is in your line of fire. You can blind fire your weapons as they approach. Keep this position until their number has dwindled.

By then, make your necessary move, either go loud and take out the remaining enemies in a firefight or continue with your stealth approach. The boss will be in the small office in the second floor. There's also a Vargas painting in the small office he's in, in case you missed it first hand.