You need to earn 20 GOLD
medals. The challenge description says 20 medals, but our esteemed community members have found they only make progress on the challenge with GOLD medals (thanks to Stealth David and HobbesOnFire). Go to multiplayer (any mode) with one of these cars (DLC packs in parenthesis):American Street Muscle
1957 Chevrolet Bel Air (Meguiar's Pack)
1967 Chevrolet Chevelle SS-396 (Meguiar's Pack)
1970 Chevrolet Chevelle SS-454
1970 Chevrolet El Camino SS 454
1964 Chevrolet Impala SS 409 (IGN pack)
1966 Chevrolet Nova SS (Smoking Tire pack)
1969 Chevrolet Nova SS 396
1969 Dodge Charger R/T (LaFerrari pack)
1970 Dodge Coronet Super Bee (Bondurant)
1968 Dodge Dart HEMI Super Stock (Meguiar's Pack)
1940 Ford De Luxe Coupe (Meguiar's Pack)
1956 Ford F-100 (Hot Wheels pack)
1968 Oldsmobile Hurst/Olds 442
1969 Pontiac GTO Judge
You might want to stay away from the Fords; they're kind of old and under-performers compared to the rest.
My preferred method is drag racing, since the races are short and done in rapid succession. However you may have a hard time suiting up your car from the 1960s to contend with the newer, faster cars. In this case, I'd recommend low-classing your car in drag racing (with a drag tune). For those of you not familiar with this method it works like this:
1. All cars in the drag racing hopper are paired together based on class starting from the top class to the low class.
1a. In an 8 player drag race, the highest 8 class cars race against each other. The rest of the cars race in a separate race.
1b. In a 2 player drag race, the highest 2 class cars race against each other, then the next 2 race against each other, and so on...
2. Even if you are the lowest class car in a drag race, you may still get a gold medal.
2a. In an 8 player drag race, you are guaranteed a gold medal if you are the lowest class car and there are 9 players in the hopper lobby when the race starts (you are put in the second batch of racers, and guaranteed a 1st, 2nd, or 3rd place medal).
2b. In a 2 player drag race, you are guaranteed a gold medal if you are the lowest class car and there are an odd number of racers in the hopper lobby when the race starts (you get put in a race of your own, and are guaranteed a gold medal).
3. Even if the scenarios in 2 above don't work, sometimes people idle, collide, or are just slower than you so you may still earn a gold medal.
Note: For drag racing I recommend turning off all assists. You won't be turning, braking, following a suggested line, or worrying about damage, so you can earn yourself a few extra credits. You can also turn on automatic transmission, put a rubber band around the accelerator, and not have to pay much attention to the races if you like (though you'll perform worse). If you do that, you may want to at least put on TCS to prevent lurching at the start.
Otherwise, you're welcome to attempt to gold medal in any of the other multi-player categories. You could try combining this challenge with other achievements, too. Just take a look at the achievement list (specifically the Nurburgring and Long Beach DLC achievements). If you want to help out your fellow community members with paint or tune credits, you can let them know in the comments below. Remember, this challenge only lasts until August 5, 2014.
Cheers and happy racing!