Keep going west until you reach a Punk Girl and a trash can. Go north. If you have learned the skating tricks from the skaters in Lumiose City (see the "Skate Tricks" section at the top of the page) The Punk Guy will send you to talk to Boss. He'll teach you the cosmic flip. To perform it, you have to jump from a ledge while doing a drift and dash. The drift and dash is performed by quickly skating in the opposite direction. So you have to skate away from the ledge, then quickly change direction toward the ledge to hop off and do a cosmic flip.