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.

Hey this is the quickest way that I found to complete online medals with the help of some fellow gamers of course, I would like to thank TheIcemanCometh,HD Addict,l PuK3 l

First you will need 1 fellow booster you can do it with 2, but 1 is best that way you can trade off on 1st place lead miles.
Now you and you friend or friends get in a party, go to multiplayer push y for private match. Once you are in match have you friend come into multiplayer and it should give them an option to push start to join your lobby. Now go to game settings and change the following:

Map: Indianapolis Oval
Max Players: 16
Number of AI: Min 2
Drivatar Difficulty: depends on you more money the higher the difficulty.
Laps:1 (or as many as you like because it add to you 1000 miles leading in first place when you coming in 1st)
Damage difficulty: whatever you like
Collision Mode: Always off (up to you I like it off)
Car class: Any (higher the class faster the finish time)
Drive type: Any (I like all wheels)

Hit RB for advanced Rules

Change End race timer at the bottom of the page to 5 sec or 10 up to you.

Now have everyone human ready up and start the race choose who comes in 1st, 2nd and 3rd if you have a 3rd, and then just rotate.

Happy boosting :)

Update: if you put 16 AI in the race the medal rewards are somewhat like single player
1st 2nd 3rd:gold
4th 5th 6th 7th:silver
7th 8th 9th 10th:bronze

Update:
I found a quicker way to get you medals you can get the track done in about 35-37 sec with a A 700 class car here is how:

Map: Top Gear West
Max Players: 16
Number of AI: Min 2
Drivatar Difficulty: depends on you more money the higher the difficulty.
Laps:1
Damage difficulty: whatever you like
Collision Mode: Always off (up to you I like it off)
Car class: Any (higher the class faster the finish time)
Drive type: Any (I like all wheels)

Hit RB for advanced Rules

Change End race timer at the bottom of the page to 5 sec or 10 up to you.

(Note this way will not get you much 1st place lead miles)