Race with Ryan

If you happen to have four controllers (like I do), you can easily cheese through all of the game's cups with little effort on your part. Simply go to multiplayer, select the cup (in this case the Royal Race Cup) select any character for you and a couple of dummy profiles, and then select the difficulty needed for the achievement (which for this one you need Medium).

This replaces all of the AI characters with player characters instead, which you can then manipulate.

Once the game starts, pause the game and on every controller hit and then . This will make every player auto drive the entire course. Then when you unpause, simply brake on the dummy profiles for a couple of seconds each with (which will give your character a head start) and then go do something else for a minute or two. The race will auto complete and you'll get your win.

When the race is over, just brake on the dummy profiles again at the beginning of the next race, and repeat as necessary.

You can use this method for every single achievement in the game aside from the drifting achievements, which you'll have to do manually.