Finish last in a Multiplayer race
This is most easily done with a single Drivatar, and must be done in Multiplayer.
From the main Forza screen, choose Multiplayer, press to create a private match and to confirm. Once the vehicle selection screen opens, choose any car you prefer. At this point you will be in the lobby, so press to change the options to match these choices:
Game Type: Drag
Track: 2-player 1/4-mile drag
Max Players: 2
Max Drivatars: 1
Drivatar Difficulty: Unbeatable
Car Class: Any
Start the race up and stay behind the other car, finishing in last place. The achievement will unlock once you cross the finish line.