Well, that's really easy. After you've beaten Cynthia, the Champion and the Elite Four, fly to Snowpoint City, and you'll be automatically be flown in front of Snowpoint's Pokemon Centre. Then you head left until you see stairs, go down the stairs, go left again, and talk to the Sailor behind the boat. But when your about to talk to the Sailor, Cynthia will pop out of nowhere and tell you that she's travelled all the way to Snowpoint just to talk with you, and blah blah blah, then once your finished talking, talk to the Sailor. He'll ask if you want to go to the Battle Frontier. Just select yes and Presto! You'll be whisked to the Battle Frontier in no time.
