To get this achievement you must get platinum times on all easy, medium and hard tracks. Tournaments, Skill Games and Extreme tracks are NOT necessary.
This achievement will take a lot of patience, dedication and skill. As for tips, I recommend the Phoenix 250cc for all 16 tracks that you'll need to do, although the Scorpion and/or Banshee will do the trick, but can be more difficult to use for higher difficulties. Another tip is to watch the replay of some of the top players in the world or a friend who might already have the platinum medal. This will be useful as it will show you exactly where to save time as well as some popular shortcuts and the best routes. It probably impossible for you to match the top riders, but having a rough idea of where they are able to shave of valuable seconds should benefit you greatly in your platinum quest.
The easy and medium tracks are much more strict on time, while the Hard tracks have a little more room for error in terms of time, as long as you manage to get 0 faults. Another tip I have is to not spend too much time on the same track in one sitting. From personal experience, repeating the same track over and over and over again usually isn't much help. I recommend that you try some other tracks when you're struggling and come back to it another time. When I found myself struggling on a specific track, I would come back to it later, and found myself getting some platinum medals on my first or second try after taking a break. It will help if you memorize each obstacle, and you give your fingers a break from doing the same motions and making the same exact mistakes.
Lastly, try YouTube. There is a lot of content support for Trials on YouTube, and I'm sure there are replays of platinum medals for the tracks available online in video form. Some of them will provide useful commentary or a slowed-down version of the replay to add insight, and these might prove valuable.