Enter the Time Attack mode and look up Ocean View. To the right of the Ocean View are times that were made by Staff Ghost. The easiest one to defeat is Easy Ghost, who had a time of 44 seconds. That's the time you should beat, so pick the stage, then chose your racer. In my case, The best characters for Time Trial are Shadow and AGES. Once the race starts, just go through the first lap of the course and when you make it to the finish line, you should have a time of 36 seconds or so, beating that ghost's time and unlocking the achievement. This shouldn't take you long to complete, about two minutes top, less even.