Forza Motorsport 5

I've just found a easy way to get those medals:
I don't know if this is a glitch or a feature of the race, but let's go...
1. Go to a normal (not private) Drag multiplayer race.
The most important:
2. Choose a car with a different class of everyone in the lobby. Players are usually using class R, X, S, P... I chose an A class car;
3. You'll race "alone". You can be the last one to get to the finish line, but you'll still get gold medal.

** EDIT: Gustavo82RS found a pattern to race alone in a full lobby... You must have the lowest class car and have to be in a lobby with an even odd number of players. Doing it, you'll race alone and therefore, win the gold medal. Thanks man!

** EDIT 2: Translation error... It's not an even number of players... It an ODD number of players (1,3,5,7,...). Really sorry about that. Thanks IzzyXboxOne!

4. Rinse & repeat.

Using this technique you can go for all Multiplayer achievements for leading, for getting gold medal and even for affinity level, but it will take a looong time for the affinity, so I think you should try the better solutions available here.

If there's any mistake, let me know and I'll improve it.