Use a class D engine to beat an opponent with a class A engine in a ranked match

This achievement is impossible in a race event unless one of the A Class riders is either really horrible, or quits out early. You'll want to try for the achievement in a Freestyle event, where it has to do less with the engine and more with the player. You do not have to win the event completely, you just have to beat a single A Class rider.

Note: It is possible to boost for this. If a friend uses the same search parameters, it's a high chance he/she will make it into your lobby. Once they are in the game, they can just sit at the starting line for a couple of seconds and wait for you to gain a head start.