This achievement is quite hard as some of the challenges are very difficult. The easiest one in my opinion is dribbling. To get legendary on this challenge you need a score of 22,500. As stated in the "Road to Mastery" choose Messi or Ronaldo. It's very easy to knock the ball out of bounds on this challenge losing 500 points each time. So make sure you're holding when going round the cones. It's also advised to do a few trial runs to memorize the course and to try and perfect each part. A few tips to get extra points is to score and finish as fast as possible. Another tip is to start over if you don't get past the defender within 1 or 2 tries.
Video Guide from Bronze to Legendary: