I recommend trying to do this with the following settings:
Race: Lv.3 - Shellfish Stairs
Character: My Avatar or Krystale
Board: Rare Board – High Top
"A" Rank reference time: 1’08”759
This is just a straight race to the bottom and the fastest time wins. Play the course a few times to get a feel for it and then get to the bottom as fast as you can by using whatever paths and shortcuts get you to the bottom the quickest. Don’t forget to use your Boost start and lean forward during the entire race to go faster. Initially I was trying to get this on the Lv.1 race and could never come close. I was about 10 seconds away from the "A" rank reference time. After switching to the Lv.3 race, I got it on the third run. You can cut off huge sections of this race by just going off course and splitting the "S" curves in half with a straight line. Even though there are more racers in this event, they stay on the course and are never even close to you. Do a mini jump and spin to get boost which will help you blast past the off course sections. Then just be sure to aim your landings so you are facing the right direction when you get back on course.