This is one of the challenges you have to complete in campaign mode. This one is sort of easy if you already did the Slalom boss achievement. Hug the flags and try and hit them while you're going down without getting a penalty. This can be hard because of all the turns. Also hold down as long as possible and use the button to carve only when you’re in a big corner. Remember, hugging the flags are important, if you don’t do this then winning is virtually impossible. To help, let the boss get in front of you and watch what he does and then try to replicate what they do on your next run.