Okay, so this won't be a perfect solution. I'm not good at drag racing, I don't have a knack for tuning, I don't use a manual transmission, and I probably don't even have the right car for this challenge, but I'm making progress on the challenge and I wanted to share how I'm doing it.
First, let me go over the basics of this challenge. On the surface this challenge seems easy. Just earn a medal in 50 Multiplayer Hopper Drag events. These are under the special events tab of the Multiplayer Hoppers, and you can choose between Unlimited or Unlimited RWD Drag racing. Then, from the lobby, you get to vote on various lengths of tracks, some of which might be 2 car, and some of which might be 8 car. To medal in a 2 car event you have to get first place. To medal in an 8 car race you have to finish in the top 3. Simple right?
Well, no... some people actually do drag racing for fun, making it a bit difficult to produce consistent results. But then there's people like me who don't care much for drag racing, but are trying them out just to get multiplayer medals. If you're lucky, you can get a decent car with a drag race tune and find yourself up against another casual and do just a little better and get a medal.
So, here's what I did:
1. Get a fast car, the fastest you can afford, and get a drag race tune on it. You can't rely on stock tunes for drag racing. It just won't work. You need drag tires at the very least. Messing around with the tuning can help, but I can't offer any real specific advice because each car is different. The car I used was the Honda Dallara DW12 Target Chip Ganassi #9. The cost is 1.3 million credits (I used it for the recently expired long beach challenge). [I managed a decent tune with it, if you want to use it search for 'xt5' in the description field of a tune search and my drag tune should pop up.] [also, I've got a paint job under 'xt5', and I'd really appreciate the extra credits from you racing with my paint job. I still need those achievements]. Otherwise, if you have a different car you can try tuning it yourself or try searching for a drag tune.
2. Get in to the UNLIMITED (not RWD) Multiplayer Hopper Drag Event. Unlimited means any class car can race (you picked a fast car, right?). I didn't really bother voting for a specific track; I didn't find one easier than the other to medal in.
3. Turn off all assists except for TCS (Traction Control System) and keep automatic transmission on. I ask you to turn everything else off just to earn some extra credits. The other assists aren't necessary; you won't be steering, braking, or following a suggested line, and you don't need to worry about damage.
4. Now, take a rubber band or hair tie and hold down the right trigger (the gas pedal) with it. You can now walk away and do something else. Just make sure the controller doesn't shut off. You might have to fiddle the right stick every once in a while to make sure you don't lose connection.
5. Your car will now race for you, and hopefully you'll get some medals. All the races advance automatically, so there's no other input needed.
...Like I said, not foolproof, but I managed to get to 50% progress in only a few hours one night. I'd say about half the races I didn't medal, but the other half I did. That's pretty good for almost no effort, if you ask me. You'll also be getting experience, credits and multiplayer medals, and you don't even have to drive.
Here's another tip from Forza Grifo: If you are paying closer attention and are about to start a 2 car race with an odd number of racers, or an 8 car race with 9-11 racers, you can try switching to a different car so you are the lowest class car in the lobby. You can be guaranteed a medal by doing this, because you will be racing against yourself in a 2-car race, or yourself and up to 2 others in an 8 car race. The way the game works is like this: In a two car race, the top 2 class cars are paired together to race, then the next two highest class cars race against each other, and so on. If there are an odd number of cars in the lobby, the lowest class car ends up racing by itself. It works similarly in an 8 car drag race. The top 8 class cars are race against each other so if there are only 9-11 cars in the lobby the lowest class cars race against each other. Try experimenting with it. Thanks Forza Grifo for the tip!
The only other option for this challenge is to be a tryhard by getting good at drag racing and tuning your car. A viable option, but it was not for me! According to Delta381 the best non-DLC car for drag racing is the Hennessey Venom, and the best DLC car is the Alfa Romeo 33 Stradale. You can start with those cars.
If you have questions or comments, please leave them below. A thumbs up will warm my heart, but if you leave a negative please let me know where I can improve.
UPDATE: Just got the challenge this morning. I kept checking on it, but every race I witnessed I didn't medal. However, I still got the challenge done so I must've medaled at least 13 times in the 1.5 hours I had it running. I must get in more races than I think is possible.